A plane intersects a prism to form a cross section that is a polygon with five sides. The minimum number of sides that the polygon at the base of

the prism must have ls ...

Since the plane if cutting through the prism to make 5 sides, than the original shape needs to have less than 5.

A prism, is a 3D shape, so you can't have a 2 sided shape.

Now you have a choice of 3+or 4 sides.

A 3 sides shape is a triangle, which also can't be cut into a 5 sided prism, so the answer would need to be 4.