Answer:
a) the dehumanizing experience of slavery
Step-by-step explanation:
The strongest evidence for me is his last line 'Thus was I like the hunted deer'. Him saying this is him comparing himself, a slave, to animals which are below human.
You are able to rule out the other answers, too.
d) the wilderness acts more like a metaphor to his central idea, than his central idea itself
c) Equiano doesn't mention the Christian faith anywhere
b) the brutality that he was treated with is implied but not illustrated. He doesn't describe how he was beat, cheated, or otherwise mistreated. We can infer he was treated with brutality because we know about the nature of slavery.
hope this helps!