Final answer:
The curvature of a closed universe resembles a sphere, following spherical geometry where initially parallel light rays converge, indicating positive curvature.
Step-by-step explanation:
The curvature of a closed universe would resemble C) A sphere. In such a universe, if one were to travel far enough in any direction, they would eventually return to their starting point, similar to circumnavigating Earth. This type of geometry is known as spherical geometry, where two initially parallel rays of light will converge, indicating a positively curved space.
An open universe is negatively curved like a saddle, leading to the divergence of initially parallel light rays. A universe with exactly the critical density is flat, where parallel rays remain parallel, exemplifying Euclidean geometry.