Translation:What are your hobbies?
Nouns don't change when used as singular or plural like in english. Either you can tell from context, or you add 一个 for example to talk about a specific thing or 一些 for a few, etc.
This sentence in particular makes no sense as singular, so pluralness is implied. If you want to ask about the single most favourite hobby, you would probably just say so and add 最喜欢.