This is the example of user after posted the question in a forum website. They need people to answer their question. I have two layout which is Version A and B. May i know which one is the best in term of UX? and why?
I'd go with A. Consider the case where a question is long. If I read the whole thing the Answer button could be above the fold. Even for short questions, my attention is at the bottom of the question, so it makes sense for the next step to be down there within my area of focus.
You might want to look at how similar sites behave. Stackexchange places the answer field (not a button, thereby removing one click) below the question.