Answer
$(d)$
Work Step by Step
$f(x)=\left\{\begin{array}{ll}
{x-5,}&{\text{if }x\lt-6}\\
{2x^{2},}&{\text{if }-6\leq x\lt-1}\\
{|x|,}&{\text{if }-1\leq x\lt 10}\\
{3x+1,}&{\text{if }x\geq 10}
\end{array}\right.$
$x=15$ belongs to the interval $x\geq 10$,
so we calculate its function value with $f(x)=3x+1$
... choice $(d)$
