Answer
Answer: B
Work Step by Step
The absolute value of a complex number
$z=a+bi,$ denoted $|z|,$ is a nonnegative
real number defined as $|z|=\sqrt{a^{2}+b^{2}}$.
$|z|=|9+12i|$
$=\sqrt{a^{2}+b^{2}}\qquad$ ...substitute $9$ for $a$ and $12$ for $b$
$=\sqrt{9^{2}+12^{2}}\qquad$ ...simplify.
$=\sqrt{81+144}$
$=\sqrt{225}\qquad$ ...evaluate.
$=15$
$|9+12i|=15$