"What are your doubts?"
Translation:Jakie są twoje wątpliwości?
You may have "wątpliwości" when you're not sure about something, e.g. when you don't know if your decision is going to be good, if your plan is going to work, if someone will like you etc.
"Zwątpienia", on the other hand, are kind of second thoughts, when you suddenly stop believing that something makes sense or you start to doubt in your success and (sometimes) despair. This word, however, is used very rarely, you'd rather use the verb "zwątpić (w coś)". For example:
Zwątpiłem. - I gave up./I let go./I stopped believing [in the point of doing something].
Zwątpiłem we wszystko. - Nothing makes sense to me anymore. (hard to translate literally)
Mam wątpliwości co do twojego planu. - I have doubts about your plan. (=I'm not sure it'll work)
Rule of thumb, which will not work in 100% but currently I am not able to give you anything better: Mostly, if your English question has "What" and some noun, you use a form of "jaki". Also, if your sentence would make sense as "What kind of", that's definitely "jaki". Example: "What book are you reading?" or as here, "What are your doubts?".
If your question just has a "what" and no noun (e.g. "What are you doing?"), you use a form of "co".
Major exception from the "noun" part: The exact form "czym" (Instrumental) will be used when you're asking for a definition, e.g. "What is love?" (Czym jest miłość?).