Common Core End-of-Course Assessment - Page 881: 49

$\bf{B}$

Here, we have $\overline{OA}^2+\overline{AB}^2=(\overline{OC}^2+\overline{BC}^2)^2$ or, $r^2+(15)^2=(r+9)^2$ or, $r^2+225=r^2+18r+81$ or, $r^2-r^2+225=18r+81$ or, $18r=144$ or, $r=8 \ cm$ Therefore, the correct option is $\bf{B}$.