What is the shape of the cross section of the cone in each situation?

When the cone is sliced parallel to the base we obtain--
Circle.
and when the cone is sliced perpendicular to the vase and passes through the vertex we get--
Triangle.
As we know that the base of a cone is in the shape of a circle.
Hence, the cross-section so obtained is a circle.
Hence, in the case of cone when the slicing is done perpendicular to the base and passes through it's vertex then we obtain:
Triangle.