A circle
The representation of this problem is shown below. The cross-section is the name for a slice that cuts through a solid. If we move around the height of the cone through its volume, we will find that at every point the cross section will be a circle. The radius of that circle will depend on the point we are on. On the base of the cone, the circle will have the same radius of the cone and the radius will be decreasing when moving until we get to the apex.