Perimeter means the total distance around the boundary. For a rectangle, perimeter = 2 × (length + breadth) = 2 × (31 + 12) = 86 mm. Notice that both measurements are lengths, but the answer is in square units because we are counting how many unit squares cover the surface. Do not add the sides here; adding sides is used for perimeter, not area. So the correct choice is 86 mm.