In answer choice E, many of "whom" - how do you know if it should be whom or who? To me who seems to be fine there and could be even better than whom.
Lets revise the basics:
Use whom = when you are referring to the object of the sentence
Use who = when you are referring to the subject of the sentence
In answer choice E: Whom is used as a relative pronoun, to refer back to the object of the sentence i.e the people. ( Population as a whole cannot be members of the hill tribe). so to make the sentence clear, whom is used.
Now you ask why whom? Because the people refer to the object. Who are members of the hill tribes? The people are
Kudos if I helped