Correct Answer
The triangle, square, and trapezoid are possible cross sections; the rectangle and hexagon are not:
A cross-section is the shape you get when you slice a 3D object with a flat plane. The shape of the cross-section depends on how the plane intersects the object.
Triangle
A triangle is made by a slice through the apex of the pyramid perpendicular to the base.
Square
A square is made by a slice parallel to the base, intersecting all four faces of the pyramid.
Trapezoid
A trapezoid is made by an oblique slice intersecting all four faces of the pyramid.
Rectangle
A rectangle is not a possible cross-section because a square pyramid has only four triangular faces meeting at the apex and the base is a square. A plane intersecting these faces will never create a cross-section that has opposite sides of unequal lengths (which is the case for a rectangle). A plane parallel to the base forms only a square, and any other plane creates shapes with more variety in side lengths, but not a true rectangle (see the trapezoid explanation that follows).
Hexagon
A hexagon is not a possible cross-section because a square pyramid only has four triangular faces plus the square base. There is no way for a plane to intersect all five surfaces in such a way that it would create six sides in the cross-section. Therefore, a hexagonal cross-section cannot be formed.