In this question, "the Oneida" functions identically to constructions such as "the French" or "the English", which are also plural. The phrase "the Oneida" refers to an entire tribe / group / race. So, A and B are eliminated.

D uses the wrong idiom 'in siding'.

Imagine the sentence: He was the only one (to win / in winning) the match. Here, obviously, the right answer will be 'to win'. By the same logic, D is wrong.

E. 'had sided' is wrong because only one past action is described in the sentence.

C: correct

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

A. was the only one of the five-nation Iroquois League who sided
B. was alone of the five-nation Iroquois League when they sided
C. alone among the five nations of the Iroquois League sided
D. were the only ones out of the five nations of the Iroquois League in siding
E. only of the five-nation Iroquois League had sided

OA C.

Please justify why A andB are wrong and why is C right. Thanks

Ah thanks.

I googled Oneida - A member of an American Indian people formerly inhabiting upper New York state, one of the Five Nations.

Makes perfect sense now.

Cheers