Each shape is made of six equal squares. Which of the shapes can NOT be folded into a closed cube?

Shape (b) has a row of four with two squares under its middle squares. Folding the row into a ring of four side faces, both extra squares land on the same end of the ring, so one end is covered twice and the other is left open. (a) is the staircase net, (c) the 3–3 net and (d) the cross: all fold into cubes.