Thank you for your help!

communication & media studies.

My favorite subject in school was detention. :P

I think human communication and therefore speech is seldom governed by strict logic and (luckily) more often driven by our personality, experience, context, and eventually roles and authority of the persons interacting. If you are answering to a QUIZ on TV, of course you should only say one subject, but if someone makes you that question in an job interview, or in a friendly environment, then you are the only good judge for deciding over a minimal or extended answer, and I think this fact to be independent from the language. Please excuse me for all these words, I am not able to make it shorter! I realize now that Chris told you quite the same!

This answer is kind of a cultural one. Grammatically you would only say one, because you were only asked for one. However, people tend to do one of two things. One: They well ask "Do I have to pick just one?" then depending on the answer give their answer. So if no, just list your favorites. If yes, then struggle to pick one haha. Two: They will list however many they want anyway. Which is fine. Most people unless there a strict reason to only give one, will understand when you give more than one. So example: A: "What is your favorite subject?" B: "Well, I like science and math." Even though grammatically that looks wrong, most people will think its perfectly acceptable.