Answer(b) is appropriate whether you ask "what subject do you like ?" or "what subjects do you like ?" The person asking the question is independent of the person who answers. The person who answers can choose to answer this form of the question either in the singular or plural. If the person had asked "Which subject do you like best ?" - that would demand a singular response.

Both sentences are correct. Say 'subject' if you're only asking for one subject. Say 'subjects' if you're asking for more than one. In this case, the question probably was 'What subject do you like?' since only one answer was given.