#### Answer

a. $a$ must be positive
b. $a$ must be negative
c. $a$ must be 0

#### Work Step by Step

a. The expression $\sqrt a$ will be positive only when $a$ is positive. For example, $\sqrt 9=3$.
b. The expression $\sqrt a$ will not be a real number only when $a$ is negative. For example, $\sqrt -9$ is not a real number, because there is no real number $b$ such that $b^{2}=-9$.
c. The expression $\sqrt a$ will be positive only when $a$ is equal to 0. That is, $\sqrt 0=0$.