Answer:
Polonius uses Ophelia as bait in order to determine the validity of Hamlet's madness.
Step-by-step explanation:
It is in Act 3, scene 1 that Polonius and Claudius are talking about how excited Hamlet is about the players' performance that night and they wonder about the motives for his madness, there is that Ophelia becomes part of the plan as they think that Hamlet will tell her his inner thoughts if they have the chance to speak "alone" while they are in fact being observed from a hiding position.