Answer: D. Square
The cross section is shown as the shaded region. There are four sides to the shaded figure, so the cross section is a quadrilateral. This rules out choice C. We can also cross choice B off the list because cross sections of 3D figures are 2 dimensional.
Choice A is ruled out as well because we have both pairs of opposite sides that are parallel. The only thing left is choice D. This is true assuming we have a square prism. Any cube is a square prism that has its height equal to the width and length.