What structure is formed by material in orbital motion around a massive central body, typically a star?

The correct answer is accretion disc. An accretion disc is a structure formed by the gravitational attraction of a massive central body, such as a star, which pulls in surrounding material. As this matter spirals inward, it accumulates in a disc shape due to its angular momentum. The motion of the material around the central body creates a rotating disc of gas, dust, and other particles.

This structure is commonly found in various astronomical environments, including around young stars, black holes, and even in the formation of planetary systems. The material in the accretion disc can eventually lead to the formation of new celestial bodies as particles collide and coalesce.

In contrast, a galaxy is a massive system consisting of stars, star clusters, gas, and dust bound together by gravity, while a planetary ring is a disc of particles and debris that orbits a planet, and a star cluster is a group of stars that are gravitationally bound to each other. Each of these structures involves different scales and contexts compared to an accretion disc, which specifically pertains to the motion of material around a central massive body.