The numbered net is folded with the numbers outside. Three of the dice (a) to (d) can be made from it. Which one cannot?

From the net, 4 is opposite 3, 1 is opposite 5 and 6 is opposite 2, and none of the dice shows an opposite pair. So check the order round the corner. (a), (c) and (d) match corners of the folded net. (b) shows 2, 5 and 4, which do meet at a corner, but turning the corner the other way: on the real cube 2, 4, 5 follow each other in that order, so (b) is a mirror image.