Answer:
Rectangular
Step-by-step explanation:
Given that a cylinder is cut by a plane.
The plane is perpendicular to the base.
Imagine a graph with a line y=r
This will be a horizontal line. When this is rotated between x=0 and x=h, we get a cylinder with radius =r and height =h.
When a plane cuts this cylinder the plane would be in xy plane a horizontal line.
So when a horizontal plane cuts the cylinder, we would get a planar closed shape which has opposite sides parallel and each angle 90 degrees.
Hence this will be a rectangle with length = h and width = r.
If h happens to be equal to r, in the special case, we may get a square.