Hey there! :D
Cluster- a group of stars or galaxies forming a relatively close association.
Constellation- A group of stars that makes a shape, and has a name that is well known.
Solar system- a system of planets revolving around a center, or sun.
Universe- the whole wide range of galaxies, stars, and solar systems that were ever formed.
The best answer would be "Cluster" because the universe is infinite, they are millions of galaxies, solar systems, and constellations in it. The question only states 'a thousand galaxies'. This is of one kind. This best works for the question.
I hope this helps!
~kaikers