Answer
The graph labeled as (c) matches with $a_n=18(-\frac{2}{3})^{n-1}$
Work Step by Step
$a_n=a_1r^{n-1}$
$a_n=18(-\frac{2}{3})^{n-1}$
$a_1=18$
$r=-\frac{2}{3}$
Since $a_1\gt0$ and $-1\lt r\lt0$, the terms of the sequence are alternating between positive and negative with their absolute value decreasing.
