OpenSAT

The diagonal of a rectangle divides the rectangle into two congruent right triangles. Since the diagonal is the hypotenuse of each of these right triangles, the Pythagorean Theorem gives us $a^2 + b^2 = (\sqrt{20})^2$, or $a^2 + b^2 = 20$, where $a$ and $b$ are the length and width of the rectangle, respectively. Since ABCD is a rectangle, the area is $ab$. Since the area is a product of two sides, the area will be a factor of 20. Of the choices, 10 is the only factor of 20, and the area of the rectangle is $ab = (2)(5) = 10$.