We have a prism.
We have many possible cross-section that are perpendicular to the base.
If we have a section that is parallel to the front face, with dimensions 15 * 7, we get the same figure: a rectangle with dimensions 15 * 7.
If we make a section parallel to the other face, a square of 7 * 7, we get a figure that is also a square of 7 * 7.
From the options given, the only shape we can get is a square.
Answer: square.
