Answer
The graph labeled as (a) 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 $0\lt r\lt1$ the sequence is decreasing and all the terms are positive.
