Answer: Right triangle
Refer to the figure below for an example.
Imagine the right triangle acts like a propeller to spin around to carve out the 3D shape of a cone. The axis of rotation is the vertical part of the triangle. In a sense, it's like a revolving door of sorts.
In contrast, a square and rectangle will rotate around to create a cylinder. An ellipse or circle rotates around to form a (squashed) sphere.