Question

A triangular pyramid is cut by a plane. in which case is the cross section a quadrilateral?

Answer 1

When you cut it directly to the middle...then the cross section will be an isosceles trapezoid... Which is also a quadrilateral

Answer 2

when the plane intersects the base and all three lateral faces of the pyramid

Explanation:

In order to get a quadrilateral, that is, a polygon with 4 sides, it is necessary that the plane intersect 4 faces of the 3d shape. A triangular pyramid has four faces, a base and three lateral faces. So, the plane have to intersect them all.