First, know what a cross section is. A cross section is the exposed shape or surface when a straight cut is made through a 3D figure. For example, if I make a cross section, or cut, through a sphere, the cross section will be a circle. The newly exposed surface is in the shape of a circle.
We want to know which 3D figures can make 2 different cross sections, parallel to the base, that are congruent. Congruent means <em>same shape but different size.</em> <em>Parallel to the base</em> means the cut is completely horizontal.
A cone is one, because no matter where you horizontally cut this, the cross section will always be a circle, but different sizes.
A triangular pyramid is another, because no matter where you horizontally cut this, the cross section will always be a triangle, but different sizes.
A square pyramid is another, because no matter where you horizontally cut this, the cross section will always be a square, but different sizes.