Which of the following describes half of a sphere?

The correct answer, which describes half of a sphere, is a hemisphere. A hemisphere is defined geometrically as one of the two equal halves of a sphere, created when a sphere is bisected by a plane that passes through its center. This results in a three-dimensional shape that retains the curved surface area and a flat circular base, distinguishing it from other geometric shapes.

In contrast, a pyramid has a polygonal base and triangular sides that converge at a point; a cylinder is defined by its circular bases and straight sides, extending vertically; and a prism has two identical bases connected by parallelogram-shaped sides. Each of these shapes has characteristics that do not align with the concept of half a sphere, which specifically refers to the hemisphere's unique qualities of curvature and division through the sphere's center.