Answer:
The best option is letter D. enemies, a process they believed allowed them to ingest the courage of their enemies.
Explanation:
We have two big problems in the original sentence: punctuation and ambiguity. Letter D is the only option that solves both problems correctly. Let's take a look at them:
1. t is reported that some native tribes used to eat the livers of their slain <u>enemies which they believed
</u> allowed them to ingest their courage. --> The underlined portion is problematic. Since there is no punctuation, it sounds as if "which" refers to "enemies", when it actually refers to the process of eating the livers.
2. they believed allowed them to ingest their courage. --> Whose courage? The enemies', of course, but "their" can have two antecedents in the sentence (enemies and tribes). It would be better to eliminate this ambiguity.
Thus, if we use letter D, we can correct the issues above:
It is reported that some native tribes used to eat the livers of their slain enemies<u>, a process they believed allowed them to ingest the courage of their enemies.</u>
