The correct answer is "Slaves made up the lowest social class".
In early muslim society (pre-Islamic Arabia), slavery was a common practice much like in the rest of medieval world. Arab slaves were taken captive to later be ransomed back, kidnapped as children who were abandoned or sold by their parents. Free people could also sell themselves as slaves, or could also serve as punishment for those who'd commit severe offenses to the law. Slaves were considered the lowest social class and were brutally punished on a daily basis.
Slavery certainly wasn't outlawed early and it did not distinguish ethnicity or race. Dhimmis was the name given to non-muslim people living in Islamic countries with legal protection, which meant they couldn't be enslaved in the first place.
Hope this helps!
