Choice B is correct. A rectangle’s diagonal divides a rectangle into two congruent right triangles, where the diagonal is the hypotenuse of both triangles. It’s given that the length of the diagonal is $5 StartRoot 17 EndRoot$ and the length of the rectangle’s shorter side is $5$ . Therefore, each of the two right triangles formed by the rectangle’s diagonal has a hypotenuse with length $5 StartRoot 17 EndRoot$ , and a shorter leg with length $5$ . To calculate the length of the longer leg of each right triangle, the Pythagorean theorem, $a^{2} + b^{2} = c^{2}$ , can be used, where $a$ and $b$ are the lengths of the legs and $c$ is the length of the hypotenuse of the triangle. Substituting $5$ for $a$ and $5 StartRoot 17 EndRoot$ for $c$ in the equation $a^{2} + b^{2} = c^{2}$ yields $5^{2} + b^{2} = {(5 \sqrt{17})}^{2}$ , which is equivalent to $25 + b^{2} = 25 (17)$ , or $25 + b^{2} = 425$ . Subtracting $25$ from each side of this equation yields $b squared equals 400$ . Taking the positive square root of each side of this equation yields $b equals 20$ . Therefore, the length of the longer leg of each right triangle formed by the diagonal of the rectangle is $20$ . It follows that the length of the rectangle’s longer side is $20$ .

Choice A is incorrect and may result from dividing the length of the rectangle’s diagonal by the length of the rectangle’s shorter side, rather than substituting these values into the Pythagorean theorem.

Choice C is incorrect and may result from using the length of the rectangle’s diagonal as the length of a leg of the right triangle, rather than the length of the hypotenuse.

Choice D is incorrect. This is the square of the length of the rectangle’s longer side.