#### Answer

a. Natural number
b. Rational number
c. Rational number
d. Irrational number

#### Work Step by Step

a. Because $\sqrt 9$ is 3 and it belongs to natural number .
b. $\frac{3}{10}$ = 0.3(3) which the decimals follow a pattern therefore it is rational number .
c. The decimal does not follow any patterm therefore it belongs to rational number group.
d. The result of $\sqrt 12$ does not follow any pattern but never ends --> It is irrational number.