A cube has 6 equal square faces. Area of one face = 50 × 50 = 2500. Total surface area = 6 × 2500 = 15000 square mm. Total surface area means the combined area of all outside faces. A cube has 6 equal square faces, so after finding one face area, multiply by 6. The answer is in square units, so the correct choice is 15000 mm².