The answer is The total area of the top and bottom squares. s = √7 cm is irrational value and we need rational value. Let's check all choices: A. The volume. V = (s)^3. If s = √7, V = (√7)^3 = 7√7 - irrational value. B. The perimeter of the front square. P = 4 * s = 4 * √7 = 4√7 - irrational value. C. The diagonal of the back square. d = s√2 = √7 * √2 = √(7*2) = √14 - irrational value. D. The total area of the top and bottom squares. A = 2 * s^2 = 2 * (√7)^2 = 2 * 7 = 14 - rational value.