I think that the D. CONE is the three-dimensional figure that will be formed when an equilateral triangle is rotated around the x-axis of the coordinate plane.
<span>A sphere is defined as the set of all points in space that are a fixed distance from a common point, the center. </span> <span>A cylinder is with two parallel congruent circular faces whose cross sections (taken parallel to these circular faces) are circular. </span> <span>A cube is a rectangular prism whose faces are squares.</span>
The vertex where the two triangles touch forms a pair of vertical angles, so that the angles on opposites sides of the vertex are congruent. You're shown that two legs of both triangles are congruent. All this tells you that the two triangles are isosceles, and furthermore that they are similar because of the congruence of their "base" angles.