Hi,

Greatly influenced by the Protested missionary Samuel Kirkland, the Oneida was the only one of the five-nation Iroquois League who sided with the colonists during the American Revolution.



The first and foremost thing to understand here is that "the Oneida" refers to the group of belonging to the Oneida tribe, a native American tribe. Hence, the Oneida is plural here. For example, The Swiss are the people who belong to Switzerland.
So the Oneida among the five-nation Iroquois League were the only people who sided with the colonists during the American Revolution.



Since "the Oneida" is plural, use of singular verb "was" is incorrect.

Now let us run the POE.
Choice A: SV number agreement error as discussed.

Choice B: Same error as A.

Choice C: Correct answer. Notice that this answer choice does away with all the debate regarding singular and plural. Verb "sided" is used for both singular and plural subject.

Choice D:
a. "the only ones out of the five nations of Iroquois League" is far too wordy than "alone among the five nations of Iroquois League".
b. Use of "in siding" is unidiomatic. We say: They are/were the only ones "to do" this and not "in doing this".

Choice E: Use of past perfect is incorrect. There is no other event than 'siding' happening in the sentence. This single action must be written in simple past tense.

Hope this helps.
Shraddha