Answer:
TRIANGLE
Explanation:
A pyramid is a solid shape that is made up of triangles, rectangles, squares and other polygons. A pyramid are classified based on the shape of its base. For example, the diagram given is a square based pyramid because of the shape of its base(which is a square). Other types includes the triangular based pyramid, triangular based pyramid and other polygon based pyramid.
If this square pyramid is cut with a plane that is parallel to the base of the pyramid, the shape of its cross section will be a triangle since the shapes that are parallel to the base are all triangles. So cutting the plane parallel to the base is similar to cutting out one side of the triangle making up the pyramid.