Answer:
I believe the answer would be C.
Explanation:
First, simply look at the roots of each answer choice. A rational number is any number that can be written as a fraction.
In the first option choice [A.], the square root of 8 is 2.8284.... a long decimal that continues for a whi;e. So, it can't be written as a fraction.
In the second option choice [B.], the square root of 24 is 4.8989.... another long decimal that continues for a while. So, it can't be written as a fraction.
In the third option choice [C.], the square root of 81 is 9. 9 can be written as a fraction [for example 27/3] So it's the correct answer.
In the last option choice [D.], the square root of 2 is 1.4142.... another long decimal that continues for a while. So, it can't be written as a fraction.
This is why C. is the right answer! I hope this helps!!!