Parallelogram
You cannot have a 2d cylinder. The 2d cross section will depend on the plane of the cross section.
It can be a square, a trapezium, a quadrilateral or a triangle - depending on the inclination of the plane which defines the cross section.
It depends on the angle of the plane of the cross section. If it is parallel to the cube's face (or equivalently, two adjacent edges) the cross section will be a square congruent to the face. If the plane is parallel to just one edge (and so angled to a face), the cross section will be a rectangle which will have a constant width. Its length will increase, remain at a maximum level and then decrease. If neither, it will be a hexagon-triangle-hexagon-triangle-hexagon (triangles when passing through a vertex).
The answer would be a right cylinder. A right cylinder is a cylinder that has a closed circular surface having two parallel bases on both the ends and whose elements are perpendicular to its base.
Triangular
You cannot have a 2d cylinder. The 2d cross section will depend on the plane of the cross section.
You cannot have a 2d cylinder. The 2d cross section will depend on the plane of the cross section.
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
It is a cross section.
The cross section of a cuboid box, bt a plane at an angle to all of its sides will be a point, triangle or quadrilateral - depending on it location. The cross section by a plane perpendicular to an axis of the box will be a rectangle.
You cannot have a 2d prism. The 2d cross section will depend on the plane of the cross section.
When a 3-dimensional (solid) object is cut by a plane, the 2-dimensional shape made where the solid object and the plane meet is the cross section.
The vertical cross section of a cylinder is a rectangle. It is created by slicing the cylinder along a plane parallel to its base. The resulting shape will have the same height as the cylinder but a width equal to the diameter of the base.
It is the cross-section of the figure by the plane.
Cross Section
It depends on the pyramid. If it is a square based pyramid, a horizontal plane will give a square cross section, a plane inclined by a rotation parallel to one of the base axes will give a rectangular cross section whereas a plane inclined by rotation along both basal axes will result in a parallelogram cross section. Not sure how you get a parallelogram from a pentagonal or hexagonal (etc) pyramid.