Final answer:
An ellipse is the conic section formed when a plane slices a double-napped cone at a slight angle without intersecting the base.
Step-by-step explanation:
When a plane slices a double-napped cone at a slight angle to the base of the cone and does not intersect the base, the conic section formed is an ellipse.
The four conic sections mentioned are the circle, ellipse, parabola, and hyperbola, and they are created by the intersection of a plane with a double-napped cone at various angles and positions.
Since the plane in question cuts the cone at a slight angle but does not intersect the base, it is not parallel to the side of the cone (which would form a parabola), nor does it intersect both naps of the cone (which would form a hyperbola), nor is it perpendicular to the axis of the cone (which would form a circle).
Therefore, the only conic section it can form under these conditions is an ellipse.