What action does "pay sb back" involve?

The phrase "pay sb back" specifically refers to the action of returning borrowed money to someone. When someone lends you money, the expectation is that you will repay that amount at a later date. This expression captures the concept of financial accountability and trust between individuals. It implies a transaction where a debt is settled, which aligns directly with the concept of repayment after borrowing.

The other choices do not accurately reflect the meaning of "pay sb back." Giving a gift involves an act of generosity without the expectation of repayment, borrowing money refers to taking funds without immediate repayment, and paying for someone's expenses indicates an act of support rather than a return of money owed. Therefore, the correct action associated with "pay sb back" is indeed the return of borrowed funds.