Final answer:
Polonius does not literally sacrifice his daughter to discover Hamlet's madness; rather, he uses her to spy on Hamlet to see if his behavior is caused by love for her, which is false.
Step-by-step explanation:
The statement that Polonius sacrifices his daughter to discover the cause of Hamlet's madness is false. Polonius does use his daughter, Ophelia, to try and understand Hamlet's behavior by arranging a situation where he and King Claudius can eavesdrop on Hamlet and Ophelia’s interaction.
However, there is no literal 'sacrifice' in the sense of her life being given up for a promise made to the king. Polonius's intent is to use Ophelia as a means to determine whether Hamlet’s apparent madness is due to his love for her.