Three of the cubes (a) to (d) can be folded from the coloured net. Which cube can NOT?

The net gives the pairs blue–black, yellow–green and red–violet, and no cube shows an opposite pair. (a) and (d) show the same three colours, blue, red and yellow, so at most one of them is right: folding the net, blue, yellow and red go round the corner in the order shown in (a), so (d) is its mirror image. (b) and (c) match other corners.