It is a rectangle.
A cylinder has a circular cross section that is parallel to its base.
A rectangular prism has a rectangular cross section whereas a cylinder has a circular cross section
A cylinder has a circular cross section, a square prism has a square cross section.
A circle, ellipse, truncated ellipse or rectangle - depending on the inclination of the cross section relative to the cylinder.
It will be a rectangle
The volume of a cylinder is the cross-sectional area of the cylinder multiplied by its length. The perpendicular cross-section of a cylinder is a circle.
The shape of a transverse cross section of a cylinder is a circle.
A cut through a right circular cylinder that is perpendicular to its altitude yields a circular cross-section. A right circular cylinder that is cut on a plane not perpendicular to its altitude but also but also not parallel to its altitude will yield an ellipse whose minor axis is the diameter of the cylinder. Trivial cases of a set of parallel lines, a single line, or the empty set occur when the cut is parallel to the altitude, externally tangent to the cylinder, or does not intersect the cylinder, respectively.
You cannot have a 2d cylinder. The 2d cross section will depend on the plane of the cross section.
A cylinder has a circular cross-section whereas a cuboid has a quadrilateral cross-section.
It is a rectangle.
A cylinder has a circular cross section that is parallel to its base.
A cylinder has a circular cross section, a rectangular prism has a rectangular cross section.
A rectangular prism has a rectangular cross section whereas a cylinder has a circular cross section
A cylinder has a circular cross section, a square prism has a square cross section.
A circle, ellipse, truncated ellipse or rectangle - depending on the inclination of the cross section relative to the cylinder.