Which one of the following figures will generate a cone when rotated about one of its straight edges ?

A cone (specifically a right circular cone) is generated when a right-angled triangle is revolved about one of its legs: that leg becomes the axis of the cone, the hypotenuse traces the slant height, and the opposite vertex sweeps out the circular base. Standard geometric sources and teaching materials state explicitly that revolving a right-angled triangle about one of its shorter sides produces a cone, and this construction is commonly used to derive cone volume and surface formulae. The other listed figures (equilateral triangle, sector or segment of a circle) do not, when rotated about a straight edge as described, produce the standard right circular cone in the same way.