Is one who errs in answering an optional religious question about the interpretation of a verse, after choosing an answer whose correctness he is unsure of, considered sinful? And does the Hadith, "Whoever interprets the Quran according to his own opinion, let him await his seat in Hellfire," apply to him?

There is no sin in answering questions about what is permissible (halal (definition)) and impermissible (haram (definition)) and the like in exams by guessing, because the context of answering in an exam differs from the context of issuing a definitive fatwa (definition). The Prophet (peace be upon him) approved of the Companions when they erred in answering his question about the tree, and he did not consider that speaking without knowledge. The blameworthy speaking about Allah without knowledge pertains to a definitive fatwa, whereas discussion and seeking understanding in scholarly gatherings are permissible.