Which of the following is irrational?

Question:
Which of the following is irrational?

(a) $\sqrt{\frac{4}{9}}$

(b) $\sqrt{\frac{4}{5}}$

(c) $\sqrt{7}$

(d) $\sqrt{81}$

Solution:

Given that

$\sqrt{\frac{4}{9}}=\frac{2}{3}$

$\frac{4}{5}=0.8$

$\sqrt{81}=9$

And 7 is not a perfect square.

Hence the correct option is.