Answer:
D. The horizontal cross-sections of the prisms at the same height must have the same area.
Explanation:
The formula for the volume of both the rectangular prism and the oblique triangular prism is:

Which is area of the base times the height, if they both have the same height then the area of the base must be the same, since the figures are different, as the base of one is rectangular and the other is a triangle, you can not have the same measurements for the width or the vertical crossed sections, since the area is distributed different.