Answer:
A circle.
Explanation:
Let us say you cut a sphere as shown in the figure, the boundary of the resulting shape you will get will be a circle. Why?
Because a sphere is the same from whichever direction you look at it., and any cross section, you take will be parallel to its diameter ( because the diameter can have any orientation), and since it will be parallel to its diameter, it will also be parallel to the cross section through the diameter. and since the cross section through the diameter is a circle, our cross section will also be a circle .
Imagine orienting the cross section through the diameter, such that it is parallel to the other cross section, and you will see what I mean.