Common Core End-of-Course Assessment - Page 876: 5

$20.25 \ ft^2$

$ST=\sqrt 2 RS \implies 9=\sqrt 2 RS$ or, $RS=\dfrac{9}{\sqrt 2}$ $RS=RT$, so we can write the area as: $Area=\dfrac{1}{2}(\dfrac{9}{\sqrt 2})(\dfrac{9}{\sqrt 2})=20.25 \ ft^2$ Therefore, the correct option is $\bf{B}$.