The inhabitants of the island were barbarians.

Answer: Option D

Barbarians : an uncultured or brutish person.
Bad : of poor quality or a low standard.
Uncivilised : (of a place or people) not socially, culturally, or morally advanced.
Cruel : wilfully causing pain or suffering to others, or feeling no concern about it.
Civilised : bring (a place or people) to a stage of social and cultural development considered to be more advanced.

Antonym of Barbarians is Civilised