I believe the correct answer is the base shape of a cylinder would be circle. a cylinder is a closed solid that has two parallel (usually circular) bases connected by a curved surface.This is the most common kind, and if someone just says 'cylinder' this is usually what they mean. The bases can however be almost any curved shape, but the most common alternative to a circle is an ellipse. The shape would then be called an 'elliptical cylinder'. The vertical cross-section of a cylinder is a rectangle, and the horizontal cross-section is a circle.
