Correct answer typically implies there is one answer to the question. However, best answer can also be used if multiple answers COULD be correct, but one is better than the others.
Ba2Ba4 is technically correct, it's just that it would reduce to BaBr2
I should certainly bloody hope so. Otherwise I wouldn't be doing a good job in regards to answering questions, would I?
Who has is the correct use of the word
Does she have? that is proably right :)