Final answer:
The solid that could result in a triangular cross section when cut by a plane parallel to its base could be a pyramid, cone, or prism, with the latter only if the base is triangular.
Step-by-step explanation:
If a solid is cut by a plane that is parallel to its base and the resulting two-dimensional cross section is the shape of a triangle, then the solid could be a pyramid, cone, or a prism. For example, a triangular pyramid, also known as a tetrahedron, when cut parallel to its triangular base, would give a smaller triangular cross section. Similarly, when a cone is sliced parallel to its base, the cross section is also a triangle. A prism, depending on its base shape, can also yield a triangular cross section when cut parallel to the base if the base itself is a triangle. It's important to note that the cross section will always be similar in shape to the base when the cut is made parallel to it.