Apaperweight in the shape of a rectangular pyramid is shown: if a cross section of the paperweight is cut perpendicular to the base and passes through the top vertex, which shape describes the cross section

When the pyramid is cut perpendicular to its base and passing through the vertex, the cross section which will be obtained will have 3 sides. Hence a triangle will be formed.

A triangle has 3 vertices. In this case one vertex will be located at the same position as is the vertex of the pyramid. The other two vertex will be located on the sides of the rectangular base of the pyramid.

None of the angle will be a Right Angle so the triangle will be an Oblique Triangle. To determine if the triangle is equilateral, isosceles or scalene we need to know the side lengths of the pyramid.