This is what I mentioned in the second part of option B)
Then all one can do is play the odds.... Is it more likely all your thoughts are internal, and you are simply 'interpreting' some of them as 'from God'? Or, are you actually receiving messages? Because remember, I have spoken to many believers of alternative faiths, whom also earnestly claim the very same contact.
And you are correct in some respect.... Whom could ever really prove as such...? But in the case for the Bible for instance, or any other spiritual claimed text, there does not even appear one piece of 'prophecy' within, which could not have been written by mere humans at the time it was written. If there instead existed details, denoting futuristic specific claims, which could not be widely translated to fit, due to their vagueness, then one could AT LEAST argue, that such messages AT LEAST came from beyond. Not necessarily from Yahweh, but at least from some external source 
So for such claimed external contact to be meaningful, it might possess very specific and very detailed parameters, which could not instead be interpreted in a multitude of ways, based upon how the reader wishes to spin it as such.... This would at least set the precedence of non-human contact...
So spiritualism then? There is long tradition within humanity of friends and relatives 'appearing' in a wide variety of ways after their deaths.
My own Grandmother told me about an experience she once had.
After my grandfather died she spent months grieving and calling out to him. Finally one night she woke up and saw him standing that the foot of the bed. He told her he was just fine and that she needed to stop calling out for him.
He said that if she really needed help with something he would be there for her. But her constantly calling for him was keeping him from do the things he needed to do.
She and I both 'knew' that was exactly the way my grandfather would have handled the situation. Did that actually happen or did she dream it?? I don't know and neither did she. But it -was- sufficient for her to move on.
I think everyone here could tell a similar story regarding people who have crossed over. But those stories are clearly different from 'God' appearing to an individual and telling them stuff.
After all, 'God' represents absolute authority. Much evil has been done because people believed 'God' instructed it to be done.
When a relative or friend who is dead makes an appearance with advice we don't give it any more credibility than we would have if they had said it before they died.
The real danger is when people believe GOD is talking to them and they have no method to verify their messages actually are from God.
I've always said that if God really wanted to talk to us there are ways in which he could verify that the message actually came from Him. The one I suggest is that if every diamond dug from the ground had a message written inside which was instantly readable in everyone's native language, it would be impossible to deny Divine authorship.
My standard so far is to accept that people honestly BELIEVE God is talking to them. But I go with the odds and think they are being deceived either by their own mind by some other mechanism.