The intended meaning of the sentence is that the 'oneida' was the only one of the five-nations Iroquois league that sided with the colonist during the American regime.
Option A- INCORRECT
In the sentence, 'Oneida' is the name of a country originating originating from the Iroquois league not a group of people, so the use of the pronoun 'who' to represent it is wrong.
Also, 'who' incorrectly modifies the 'Iroquois' league than the 'Oneida'
Option B- INCORRECT
There's a noun and pronoun disagreement. The pronoun 'they' is supposed to be used for plural nouns, but in this case the sentence has a singular subject 'Oneida'
Option C- CORRECT
This Option is correct because the verb 'sided' agrees with the subject 'Oneida'. Again using 'alone' in the sentence correctly conveys the writer's intended meaning that only Oneida sided with the colonist.
Option D- INCORRECT
The subject in the sentence is a singular subject and it can only take a singular verb, so the use of 'were' to represent the singular subject 'Oneida' is illogical
Option E- INCORRECT
The incorrect use of the past perfect tense 'has sided' is wrong. Past perfect tense are used to convey an earlier action in the past and there are no earlier action in the past in this sentence.
also, the use of the adverb 'only' cannot convey the intended meaning of sentence itself. The most appropriate expression should have been 'the only one'