When you draw a diagonal between two vertices of a rectangle, two triangles are formed. So to find the length of the diagonal, find the hypotenuse of one of the triangles:
a^2 + b^2 = c^2
8^2 + 15^2 = c^2
64 + 225 = c^2
c^2 = 289
c = 17 feet (option D)