Ok, wow, that really depends on the context as they all mean different things: Do/Complete are both ok (Contrary to another answer). We might use them when a teacher gives an assignment/task to students (I guess you mean complete and not compete: we couldn't use compete) "You need to do these questions for homework" "You need to complete these questions before you can go home" Finish/start absolutely Answer, of course. As Nathan says, this is the most common collocation.

Correct: - "Answer the questions." (The best choice) - "Start [answering] the questions." (An okay choice) - "Complete these questions." (This also works, but it's awkward) - "Finish [answering] the questions." (Not the best choice) Incorrect: - "Do the questions."

Answer is the most natural way to say. Compete and do are wrong, start and finish are a bit awkward and would better be used with a v+ing, like in "Start/finish answering the questions" (which could be said respectively at the beginning and short before the end of an exam).

Sorry, I put a comment instead of an answer by mistake. Please check my answer instead.