A huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity.

• A. Universe
• B. Galaxy
• C. Solar system
• D. Nebula

Answer: (B)

A galaxy is a huge gravitationally bound collection of many components—stars, star systems, star clusters, dust, and gas—that all move together as one system. That description fits a galaxy perfectly: it’s not just a single star or a cloud, but a vast assembly where gravity holds everything in a common structure. Within a galaxy you can find billions of stars, various star-forming regions, and large amounts of gas and dust that make up the interstellar medium.

Think of it this way: a solar system is just one star and its planets, so it’s much smaller and not a network of many objects bound as a whole. A nebula is primarily a cloud of gas and dust, some of which may give birth to stars, but it’s not a large bound collection of many stars and clusters. The universe, while containing galaxies, is the entire expanse of space, time, matter, and energy, not a single bound system of this kind.

So the term that best matches a huge, gravity-bound group of stars and material is a galaxy.