Please read this to and you will get the answer to your questions.. Nice question btw..

http://www.urch.com/forums/faqs/3978-gmat-sc-s-v-agreement-one-people-who-bake-bakes.html

Oh God!

I posted this question because I was not very certain about the concept of this question.

But in the end, after having 3 pages long discussion I am yet to figure out which rules apply and which rules don't.

Now I am more confused than I was before posting this question.

@Ketkoag : I am sorry friend. I am totally confused now.Please go through the thread discussion again from the beginning till this post and would be there where I am now

Source : Old paper Tests by ETS.

The OA is B and the source can be trusted.

this is the problem with people randomly posting questions without answers. ALWAYS POST THE ANSWER ALONG WITH ANY QUESTION. always always always.

this is absolutely necessary. ideally you should use the "spoiler" function, or put a little space between the question and the answer, but you MUST post the answer given in the source.
i've seen lots and lots of threads, like this one, in which a solid majority of the posters have the wrong answer. we need to counteract this trend by posting answers to all questions

this is one of our (mgmat) questions. the answer is (b).

here's a reference to another thread in which i posted the correct way to resolve these sorts of things:
http://www.beatthegmat.com/one-of-the-t40356.html#167563

the rules i cited in that post do not have exceptions.

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this is the problem with people randomly posting questions without answers. ALWAYS POST THE ANSWER ALONG WITH ANY QUESTION. always always always.

this is absolutely necessary. ideally you should use the "spoiler" function, or put a little space between the question and the answer, but you MUST post the answer given in the source.
i've seen lots and lots of threads, like this one, in which a solid majority of the posters have the wrong answer. we need to counteract this trend by posting answers to all questions.

From next time onwards I would edit the original question with the OA after few responses.That would be beneficial for all.

Thanks All.

Based on the above discussion and on other thread, I suppose that the set of rules becomes like:

1. One of the Xs that/who <plural>
2. One of the Xs <singular>
3. only One of the Xs <singular>
4. only One of the Xs that/who <plural>
5. "the" only One of the Xs that/who <singular>

Please tell if I am missing something here.

Mohit, I was thinking to start up a thread where we can all have these sort of rules at one single place. I need your advice on that.

However, we can strictly omit the basic concepts and focus on the rules and exceptions which we find in GMAT exam.

Need your Opinion.