A straight line.
A tetrahedron.
A triangular prism can have a square or triangular base.
the base of a triangular pyramid is triangle
the base of a triangular pyramid is triangle
If it is a pyramid, then the base shape is a SQURE. If it is a tetrahedron, then the base shape is a triangle.
Triangular Pyrimid
A triangular prism can have a square or triangular base, but faces are normally triangles. (I think)
A pyramid.
That would be a triangular pyramid.
It is a pyramid
A pyramid with an irregular pentagon base.
It is normally rectangular in shape.