All of the following are irrational except
a. the square root of 5
b. the square root of 7
c. the square root of 9
d. the square root of 11

C.The square root of 9

Step-by-step explanation:

A number that can not be expressed in the form of

Where, q ≠ 0, is called irrational number.

Also, the square root of a prime number is always an irrational number,

5, 7, 11 are prime numbers,

⇒ √5, √7 and √11 are irrational number,

Also, √9 = 3 which is a rational number,

Hence, option C is correct.