Which of the following refers to a collection of star systems?

A collection of star systems is referred to as a galaxy. Galaxies are massive structures that consist of billions or even trillions of stars, along with gas, dust, and dark matter, all bound together by gravity. The Milky Way, which is the galaxy containing our solar system, is just one example among the estimated 100 billion galaxies in the observable universe.

Other options describe different celestial arrangements. A planetary system specifically refers to a star and the celestial bodies that orbit it, such as planets and moons. A solar system is a type of planetary system that includes a star (like our Sun) and the objects that orbit it. A star cluster is a group of stars that are physically close to one another, usually bound by mutual gravitational attraction, but it is not as large or encompassing as a galaxy. Thus, the vast and complex organization of stars within a galaxy provides the best fit for the description of a collection of star systems.