Answer: 1) Polonius is sacrificing his daughter to trick Hamlet.
In Act 2, scene 2, Polonius wants to find out whether Hamlet is indeed in love with Ophelia. He decides to use her as bait, and hide, to see Hamlet's reaction to her. However, the plan is discovered by Hamlet, who accuses him of being like Jephthah.
This is a reference to the Bible. Specifically, Judges 11:31. In the story, Jephthah promised to sacrifice whatever first comes out the door when he is back home in exchange for victory in battle. The first person to come out and greet him is his daughter. Hamlet is implying that Polonius is sacrificing Ophelia in the same way.