Which of the following is a basic building block of galaxies?

Stars

Gas

Dust

All of the above

The correct option is;

All of the above

Explanation:

A galaxy is a massive system of stars (and their solar systems), dust, dark matter, stellar remnants, and interstellar gas held together by gravitational forces. The structures, histories and shape have been made known by Hubble's telescope in detail such that the interrelations of galactic groups and coordination of lone galaxies are detected

Examples of galaxies are Andromeda, Comet, Bode's, Cosmos Redshift, Black Eye, and the Milky way galaxy.