Hello,
Cheetah is definitely the mammal, and it was considered the fasted animal on land.
Let's see why all other three options are not correct
Cheetah could not be a reptile because they are not cold blooded animal which could regulate their inner temperature the same like the environment
Cheetah could not be a bird because it could not have any wings to fly
Cheetah could not be a fish because it could not able to swim with those 4 big legs, and they also could not have any fins to help them to do it. Hope it help!