What is the answer and how did you figure out the answer

Third option is correct.

Step-by-step explanation:

When a plane intersects one cone of a double-napped cone such that the plane is parallel to the generating line. (here, slant height), it forms parabola.

Because  a parabola is  generated by the intersection of the regular cone with a plane.

It intersects the one cone of a double napped cone and it becomes parallel to the slant line.

Hence, third option is correct.