Area of a rhombus = 1/2 × product of diagonals. Therefore, area = 1/2 × 80 × 10 = 400 square mm. In a rhombus, the diagonals divide the shape into four right triangles, which leads to the formula 1/2 × d₁ × d₂. Use the two diagonals, not the side length, for this area formula. So the correct choice is 400 mm².