#### Answer

$C$

#### Work Step by Step

RECALL:
$f(a)$ is the value of the function $f(x)$ when $x=a$.
Thus, $f(3)$ represents the value of the function $f$ when $x=3$.
Since $y=f(x)$, $y$ is the independent variable, and $x$ is the dependent variable, then, $f(3)$ is the value of the independent variable when the value of the dependent variable is $3$.
Therefore, the answer is Option $C$.