Two of the notebooks_______ Musa had lost on the bus were returned to the main desk at his room.
Two of the notebooks_______ Musa had lost on the bus were returned to the main desk at his room.
Explanation
We use which in questions as a determiner and interrogative pronoun to ask for specific information.
We use both which and what to ask questions. We use which when there is a restricted range of answers. We use what more commonly when the range of answers is not restricted