The right sentence would be

Les aiderez-vous à y répondre ?

Note that the corresponding affirmative sentence is “Vous les aiderez à y répondre”. To form the corresponding interrogative, you only have to move the subject vous after the verb aiderez and tie them with a dash. Les is an object of aiderez and isn’t affected by the process.

By the way, y only stands for au questionnaire so you still need the preposition à.