How many of the four shapes, each made of six equal squares, can be folded into a closed cube?

(a) and (c) are both 2–3–1 nets: two squares on top, a row of three below, and one square underneath; both close up into a cube. (b) is a 2 by 3 rectangle: its middle squares would have to fold onto squares already used, so two faces overlap. (d) has five squares in a line, and a cube can never have more than four faces in a ring. So 2 shapes work.