Which type of cross section is formed when the cone is intersected by a plane containing the axis of rotation?

Well we need to see the axis of rotation there must be a picture or something to depend on since you provided very few information or whatsoever.

If your picture of information has the cone cut from the middle then it's a circle and if it's from top to bottom it's an isosceles triangles.

Here's a picture I found on the internet to help you understand it better ^-^

Sorry if I didn't help but Good luck!