a) who
b) where
c) which
d) when
The correct answer is:
c) which
Explanation:
The sentence is: We saw a cat __ was sleeping on the wall.
For animals/things, we use “which” (or “that”).
Who → used for people.
Where → refers to a place.
When → refers to time.
So the correct sentence is: We saw a cat which was sleeping on the wall.