SAT math practice question: Functions and real values

 
Try the following SAT math question that tests an understanding of when functions are defined.

For what values of $a$ is the expression $\sqrt{9-a^2}$ a real number?

1. $\quad |a| \geq 9$
2. $\quad |a| \leq 9$
3. $\quad |a| \geq 3$
4. $\quad |a| \leq 3$