(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.