When a plane intersects a double-napped cone such that the plane is perpendicular to the axis, the conic section formed is a circle.
A double-napped cone is a cone that has two identical, symmetrical, curved sides that meet at a common point called the vertex. The axis of a double-napped cone is a straight line that passes through the vertex and the center of the base.
When a plane intersects a double-napped cone, the conic section formed will depend on the angle between the plane and the axis of the cone. If the plane is perpendicular to the axis, the conic section formed will be a circle. If the plane is not perpendicular to the axis, the conic section formed will be an ellipse, a parabola, or a hyperbola.
In this case, the plane is perpendicular to the axis of the cone, so the conic section formed is a circle.