The horizontal cross section of a triangular pyramid, or tetrahedron, is a shape that varies depending on the height at which the section is made. At different heights, the cross section will be a triangle, with its size and proportions changing as you move up or down the pyramid. If the section is made at the base, the cross section will equal the triangular base itself, while sections taken higher up will be smaller triangles. The shape remains triangular throughout, but its dimensions shrink as you ascend towards the apex.
It is a rectangle which is similar to the base.
A prism can have a triangular cross-section with a rectangular base
Pyramid, triangular prism, cone.
The horizontal cross-section of a pyramid forms a square when the pyramid has a square base and is sliced parallel to that base. This is because all points on the cross-section are equidistant from the center of the base, maintaining the same proportions as the base itself. As the cut is made at any height, the resulting shape remains a square, regardless of the height of the slice. If the pyramid's base were a different shape, the cross-section would reflect that shape instead.
It depends on the pyramid. If it is a square based pyramid, a horizontal plane will give a square cross section, a plane inclined by a rotation parallel to one of the base axes will give a rectangular cross section whereas a plane inclined by rotation along both basal axes will result in a parallelogram cross section. Not sure how you get a parallelogram from a pentagonal or hexagonal (etc) pyramid.
triangle
It is a rectangle which is similar to the base.
a square
A triangular prism has a uniformed cross-section whereas a rectangular pyramid does not.
A prism can have a triangular cross-section with a rectangular base
a square
Pyramid, triangular prism, cone.
a triangular pyramid
A solid with a triangular cross section when the cross section is taken parallel to the base could be either a triangular prism, a triangular pyramid, or a triangular frustrum.. I've been sitting here trying to convince myself (without actually bearing down and trying to prove it mathematically) that any cross section of a regular tetrahedron (a special case of triangular pyramid) taken perpendicularly to the base is a triangle, and I THINK this is the case, but as I said I certainly haven't rigorously proven it; I'm just unable to come up with any obvious situation where this is not true.
It depends on the pyramid. If it is a square based pyramid, a horizontal plane will give a square cross section, a plane inclined by a rotation parallel to one of the base axes will give a rectangular cross section whereas a plane inclined by rotation along both basal axes will result in a parallelogram cross section. Not sure how you get a parallelogram from a pentagonal or hexagonal (etc) pyramid.
cone, cylinder, rectangular prism, square pyramid and triangular prism
A triangular prism has a triangular cross-section. A rectangular prism has a rectangular cross-section.