“Pay off” is a B2 level phrasal verb with a couple of different meanings. Firstly, it can mean to pay all of the money that you owe. Secondly, it can be used to talk about something that you have done to try to achieve something that ends up being successful.
Thos piano lessons have really paid (3)……, she plays so well now.
After ten years he managed to pay (6)……. all his loans.