A galaxy cluster would be pretty much exactly how it sounds, a cluster of galaxies grouped together. So the answer would be D. A group of galaxies bound together by gravity. I hope this helps :)
I think the correct answer from the choices listed above is option A. When crossing-over takes place, chromosomes exchange corresponding segments of DNA. Corresponding genes on non-sister chromatids are exchanged at chiasmata. Hope this answers the question.
