Question

question 4
according to current understanding, what is a quasar?
a very large & bright galaxy thought to be formed by the merger of several smaller galaxies, typically found in the center of a galaxy cluster
an active galactic nucleus powered by an accretion disk around a supermassive black hole
a nearby galaxy with an unusually high rate of star formation

Explanation:

Quasars are active galactic nuclei. They are powered by accretion disks around supermassive black - holes, emitting vast amounts of energy.

Answer:

an active galactic nucleus powered by an accretion disk around a supermassive black hole