A net is pretty much a three dimensional shape unfolded into a flat construction. Imagine cutting the edges of a box and unfolding it so you can lay it flat on the ground. That would form a net for the 3D shape of the box.
In this case, 1 and 2 obviously aren't right since they would both involve triangles in their net. 3 is close, but the parts of the net wouldn't be completely equal. Therefore it has to be 4, the cube.
