Total surface area of a cuboid = 2(lb + bh + lh). Here lb=342, bh=42, and lh=399. So TSA = 2 × (342 + 42 + 399) = 1566 square m. A cuboid has three pairs of equal rectangular faces: lb, bh, and lh. We add these three face areas and multiply by 2 to include the opposite faces. The answer is in square units, so the correct choice is 1566 m².