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) Same
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

I picked A because I thought Oneida was singular and "was" corresponded. The answer is C and I am not sure why C is correct and A is incorrect. Thanks!

The phrase "the Oneida" refers to an entire tribe / group / race. It is used in the same way we'd say "the Americans" or "the French." They're trying to trick us b/c it doesn't end with an "s."

is "WHO" correctly used in A?

If it's a nation I think use of who is wrong it should be which/that.

Am I correct?

'who' would be appropriate, because, as stacey has noted, 'the oneida' is analogous to 'the americans'. they're a group of people.

this gets complicated with native american tribes. for example,
* 'the iroquois nation' is an 'it', not a 'who', but
* 'the iroquois' is a plural 'who', for exactly the same reason cited by stacey above.

if you aren't from the u.s., i can understand your confusion. do try to internalize the patterns you see here, though; the gmat loooooves questions involving gratuitous facts about various minorities, and native american tribes certainly qualify for inclusion in that category.

hope that helps.

What are the errors in option "D" and "E".

isn't the Oneida, like the Americans or the French, a COLLECTIVE NOUN and thus should be paired with a singular verb, WAS?

Try it out. Would you say "the Americans is here" or "the Americans are here"? "The French is here" or "the French are here"?

These are not collective nouns in the way you are thinking - you're thinking about "team" or "army" or "country" - although these nouns are made up of many members in the group (and, hence, you'd say "members of the team are"), there is just one team, one army, one country. When you say "the Americans," you are actually talking about the members, plural.