The dotted net at the top is folded into a die. Which of the dice (a) to (d) can NOT be made from it?

In a run of four squares that turns like a Z or a staircase, the two end squares fold onto opposite faces: 4–1–5–6 gives 4 opposite 6, 1–5–6–2 gives 1 opposite 2, and 5–6–2–3 gives 5 opposite 3. Die (c) shows 4 and 6 side by side, so it is impossible; (a), (b) and (d) all fold correctly.