Which three-dimensional figure has half the number of vertices as the rectangular prism?

1. The rectangular prism is a right prism with rectangular bases (check the picture)

2. It has 8 vertices, so we are looking for 3d figures with 4 vertices.

3. A face can have at least 3 vertices, and the 4th must be outside of the plane determined by the first 3 vertices. This makes 4 vertices in total.

4. The 3d figure described in step 3 is a pyramid.