Which of the following is irrational?

ANSWER

(a) \[\sqrt {\frac{4}{9}} = \frac{3}{2}\] which is a rational number.
(b) \[\frac{{\sqrt {12} }}{{\sqrt 3 }} = \frac{{\sqrt {4 \times 3} }}{{\sqrt 3 }} = \frac{{2\sqrt 3 }}{{\sqrt 3 }} = 2\] Which is a rational number.
(c) \[\sqrt 7 \] is a irrational number.
(d) \[\sqrt {81} = 9\] which is a rational number.
Hence, (c) is the correct answer.