The correct answer is: B. uncivilised.
A barbarian is a person who is considered to be uncivilized or primitive. The word barbarian is often used to describe people who are from a different culture or who have different customs than the speaker. In the context of the question, the inhabitants of the island are described as barbarians because they are not civilized in the same way as the speaker. They may have different customs, beliefs, and ways of life.
The other options are incorrect because they do not accurately describe the inhabitants of the island. Option A, bad, is incorrect because it is possible for people to be uncivilized without being bad. Option C, cruel, is incorrect because it is possible for people to be uncivilized without being cruel. Option D, civilised, is incorrect because the inhabitants of the island are not civilized in the same way as the speaker.
It is important to note that the word barbarian is often used in a derogatory way. It is important to be respectful of other cultures and to avoid using language that could be offensive.