On a real die, opposite sides add up to 7. Which of these dice is impossible?

Answer: D

Opposite faces can never be seen at the same time, so no two visible faces may add to 7. Die D shows the 6 right next to the 1, and 6 + 1 = 7, so those faces should be on opposite sides. A (4, 5, 6), B (2, 4, 6) and C (1, 4, 5) are all fine.