Question

which intersection forms a circle?
a plane intersects only one nappe of a double - napped cone, and the plane is perpendicular to
the generating line of the cone.
a plane intersects only one nappe of a double - napped cone, and the plane is perpendicular to
the axis of the cone.
a plane intersects only one nappe of a double - napped cone, and the plane does not intersect the
vertex.
a plane intersects both nappes of a double - napped cone, and the plane is parallel to the
generating line of the cone.

Explanation:

When a plane intersects only one nappe of a double - napped cone and is perpendicular to the axis of the cone, the intersection is a circle. If the plane is parallel to the generating line, the intersection is a parabola. If the plane intersects both nappes, the intersection is a hyperbola. If the plane intersects only one nappe and is not perpendicular to the axis (but not parallel to the generating line), the intersection is an ellipse.

Answer:

A plane intersects only one nappe of a double - napped cone, and the plane is perpendicular to the axis of the cone.