Answer: The answer is (B). An isosceles triangle.
Step-by-step explanation: In the given figure, a cone and its axis of rotation is shown. We are to select the type of cross-section that will be formed if the plane passes through the axis of rotation.
See the attached figure, if a plane passes through the axis of rotation of the cone, then the resultant cross-section will be a triangle with one vertex as the vertex of the cone, i.e., A. And also, the two sides of the triangle through the vertex A will be equal.
The cross-section is also shown in the attached figure and it will be an isosceles triangle ABC, where AB = AC.
Thus, the correct option is (B) an isosceles triangle.
