An ellipse or a rectangle, depending on how you cut it.
The cross section of cylinder along its length (parallel to the axis of symmetry has the shape of a rectangle.
You cannot have a 2d cylinder. The 2d cross section will depend on the plane of the cross section.
A square.
a circle
a rectangle.
A square.
If the cylinder is standing on its flat face, the horizontal cross section is a circle. Otherwise, it is a line or a rectangle.
It will be a rectangle
A cylinder is a three dimensional shape with a uniform circular cross section and a finite length.
In Geometry, cross-section is the shape made when a solid is cut through by a plane. The cross section of a circular cylinder is a circle. * * * * * There are also cross-sections that are ellipses or rectangles.
circle just did it on apex
an oval hemispere * * * * * The cross section must be 2-dimensional so it cannot be a hemispere - or hemisphere, even. I suggest the shape is lenticular: the shape of a convex lens (with equal radii of curvature).