What do you think is the answer that Paul had in mind to the question: "For who has resisted the will of God?"
Its a rhetorical question first and foremost. Secondly, you misquoted it. The verse actually states:
Romans 9:18 "Will you say to me then, "why does he still find fault? For who can resist his will?" which is predicated by the context where this verse is found which is about predestination. You have to go further back in the text to see what Paul's line of thinking is and it is clearly about God's predestination. In this chapter, Paul shows us that God created two types of vessels. Some for his wrath and some for his mercy. He says this in verse 22 but he shows these things before he actually says it. Pharaoh's predestination as Pharoah wherein God shows Pharoah is a vessel of his wrath. A vessel he raised up to be that vessel of wrath. God appointed him a Pharaoh and via pharaoh's actions he received God's justice (his wrath aka the plagues) for his treatment of God's chosen people. The elder served the younger(Esau/Jacob). God has mercy upon whom he has mercy and justice (what compassion actually means in this context) upon whom he has justice. He has mercy on whom he has mercy and he hardens whom he hardens. But let's go back and look.
Read this link before going further as it describes the context IN DETAIL prior to the verse in question:
The Argument of Romans 9:14-16 | Monergism
Now let's look at verse 19 with John Gill's Commentary
Thou wilt say then unto me,....
That is, thou wilt object to me; for this is another objection of the adversary, against the doctrine the apostle was advancing: and it is an objection of a mere natural man, of one given up to a reprobate mind, of an insolent hardened sinner; it discovers the enmity of the carnal mind to God; if is one of the high things that exalts itself against the knowledge of him; it is with a witness a stretching out of the hand against God, and strengthening a man's self against the Almighty; it is a running upon him, even upon the thick bosses of his bucklers; it carries in it the marks of ill nature, surliness, and rudeness, to the last degree:
why doth he yet find fault?
The objector does not think fit to name the name of "God", or "the Lord", but calls him "he"; and a considerable emphasis lies upon the word "yet": what as if he should say, is he not content with the injustice he has already exercised, in passing by some, when he chose others; in leaving them to themselves, and hardening their hearts against him, and to go on in their own ways, which must unavoidably end in destruction; but after all this, is angry with them, finds fault with them, blames, accuses, and condemns them, for that which they cannot help; nay, for that which he himself wills? this is downright cruelty and tyranny. The objector seems to have a particular regard to the case of Pharaoh, the apostle had instanced in, when after God had declared that he had raised him up for this very purpose, to make known his power, and show forth his glory in all the world, still finds fault with him and says, "as yet exaltest thou thyself against my people, that thou wilt not let them go?" Exodus 9:17; and yet he himself had hardened his heart, and continued to harden his heart, that he might not let them go as yet; and when he had let them go, hardened his heart again to pursue after them, when he drowned him and his host in the Red sea; all which in this objection, is represented as unparalleled cruelty and unmercifulness; though it is not restrained to this particular case, but is designed to be applied to all other hardened persons; and to expose the unreasonableness of the divine proceedings, in hardening men at his pleasure; and then blaming them for acting as hardened ones, when he himself has made them so, and wills they should act in this manner:
for who hath resisted his will?
This is said in support of the former, and means not God's will of command, which is always resisted by inability more or less, by wicked men and devils; but his will of purpose, his counsels and decrees, which stand firm and sure, and can never be resisted, so as to be frustrated and made void. This the objector takes up, and improves against God; that since he hardens whom he will, and there is no resisting his will, the fault then can never lie in those who are hardened, and who act as such, but the claim to fault is squarely on God; and therefore it must be unreasonable in Him to be angry with, blame, accuse, and condemn persons for being and doing that, which he himself wills them to be and do. Let the disputers of this world, the reasoners of the present age, come and see their own faces, and read the whole strength of their objections, in this wicked man's; and from whence we may be assured, that since the objections are the same, the doctrine must be the same that is objected to: and this we gain however by it, that the doctrines of particular and personal election and reprobation, were the doctrines of the apostle; since against no other, with any face, or under any pretense, could such an objection be formed: next follows the apostle's answer.
20 But who are you, O man, to answer back to God? Will what is molded say to its molder, “Why have you made me like this?” 21 Has the potter no right over the clay, to make out of the same lump one vessel for honorable use and another for dishonorable use? 22 What if God, desiring to show his wrath and to make known his power, has endured with much patience the vessels of wrath prepared for destruction, 23 in order to make known the riches of his glory for vessels of mercy, which he has prepared beforehand for glory— 24 even us whom he has called, not from the Jews only but also from the Gentiles? 25 As indeed he says in Hosea,
“Those who were not my people I will call ‘my people,’
and her who was not beloved I will call ‘beloved.’”
26 “And in the very place where it was said to them, ‘You are not my people,’
there they will be called ‘sons of the living God.’”
Predestination is all throughout that chapter. In fact from about Romans 8:28 to the last verse in chapter 11 is about God's elect. The reality is Predestination is found all throughout the bible.