Answer with explanation:
Pyramid and Plane both are three dimensional shapes.
Pyramid is a three dimensional shape having base any polygon and other faces as triangles.
The base of pyramid is horizontal.
1. If lower base is a Square,or rectangle or any other parallelogram
And, then , plane cuts the pyramid vertically
It will be also, a three dimensional shape ,resembling a triangular cross section in three dimension.
Triangle is appropriate choice.