Which of the following is the best definition for stream of consciousness?

Stream-of-consciousness is a very stylistic form of free indirect discourse. It is not spontaneous, or unintentional, or anything of the sort. In fact, if anything, it's just the opposite. It's highly stylized, but also purposeful and calculating. It sees the world wholly through the character's mind instead of through their senses, save for how the mind and the senses interact.

It relates to a lot of things - free association, synesthesia, free indirect discourse, without actually being any of them.

There's only a handful of writers that can actually do stream-of-consciousness writing with any success - Joyce and Faulkner come to mind immediately. In short, there's nothing wrong with trying it, but there's also nothing wrong with not having done that, but having done, say, free association instead.