Answer: B. $15$

Explanation:

$1$ . Understand the task:

   - We need to evaluate the function g at an input value of $x = 3$ .

$2$ . Substitute the input value into the function definition:

   - The function is $g(x) = 18 - x$ .

   - Replace x with $3$ : $g(3) = 18 - 3$.

$3$ . Calculate the result:

   - $18 - 3 = 15$ .

$4$ . Conclusion:

   - The value of $g(3)$ is $15$, which corresponds to option B.