Investors in the project, one who is Russian, will take part in more t

OFFICIAL EXPLANATION




• Major concept tested: subgroup modifiers
→ Although subgroup modifier SC questions are fairly rare, the subgroup modifier construction shows up frequently in RC.

For most native speakers, these turns of phrase make perfect sense.

Full disclosure: this construction is idiomatic.
Yes, there are grammar rules. See Notes, below.
Honestly, it's easier just to become familiar with the way that subgroup modifiers are constructed.

We want to talk about a part of a group.
In this sentence, we want to highlight the fact that one of the investors is Russian, presumably to explain how a bunch of foreigners got the chance to invest a crap ton of money in more than 30 projects . . . in Russia.

The subject of the main clause is a big group: "investors."
We want to talk about part (in fact, about one member) of that whole group.

(Sidebar: subgroup modifiers can also be used when we want to give more information about or to clarify something about the whole group.)

To spotlight the Russian person in this group of investors, we need both to give detail about the member we are highlighting and to keep the group to which she belongs in the background picture.

In other words, in English, when we want to identify an individual from among a large group, we must refer to the group in order to modify it.

We use phrases such as one of whom, a few of which, both of whom, neither of which, etc.
→ in the ____ OF _____ pattern, in the blanks, the first word refers to the part of the group. The second word refers to the whole group.


Rather than replicate a very similar explanation to a very similar question, I'll just direct you to it, here..

• Meaning?
Split up the sentence.
→ The main clause: Investors in the project will take part in more than 30 projects in the Kaliningrad Region . . .

→ The extra information: One of those investors is Russian.

A momentary spotlight shines on one member of the group of investors.
In order to spotlight the subgroup (the one person), the group must both remain in the background and be referenced.
We need to know that this Russian person is one of the investors.

The word "of" is key.

1) The noun part of the reference will almost always be in this form:
_____ of ______
[part] OF [reference to group]

2) If the pronoun that refers to the group is a relative pronoun (who, whom, which), the modifier must contain a verb.
→ The wild mushrooms, a few of which contained poison, populated the forest floor.
a few = reference to the part we want to identify
of which = reference to the whole group wild mushrooms
verb that must exist in the relative clause = contained

3) If the reference to the group is a regular pronoun (them), the modifier must be

(a) followed by an adjective that is often a participle (a verbING)
→ The wild mushrooms, some of them containing poison, populated the forest floor.
OR
(b) preceded by a conjunction in order to form a second independent clause
→ The wild mushrooms in the forest looked edible, but some of them contained poison.

THE OPTIONS in a shortened sentence


• this option does not follow the [part] OF [whole] pattern. It is therefore suspect.
• one who is Russian fails to reference the group from among which this one investor is highlighted
→ the singular verb is must refer to the numerical pronoun one
→ one . . . what? Yes, we know that the phrase is intended to refer to the group, but to a native ear, this construction sounds strange.
I stopped to re-read, wondering whether one referred to project because my tired brain was not focused on who. (Who refers only to people.)

• the diction is a train wreck.
→ That is, in this sentence, it seems as if the author simply plunked one who is Russian between the main subject and verb.
→ don't look for fancy appositives in order to justify keeping this answer.
On the GMAT, appositives are fairly straightforward and typically define a noun about which we know little.
The Don Juan Pond, a small and very shallow hypersaline lake located in Antarctica, is the saltiest body of water on earth.
The phrase a small and very shallow hypersaline lake located in Antarctica is an appositive of the Don Juan Pond.
Official example, SPOILER ALERT:


• Idiom error. When we participate in something en masse, we say that we "take part."
Take part = participate
Take a part = accept a role in a stage production
Correct: Every neighborhood child will take part in the community project.
Wrong: Every neighborhood child will take a part in the community project.

ELIMINATE A


• a verb or a participle (verbING) is missing
→ one of them is a subject modified by the relative clause who is Russian
→ so
who takes the verb is
the word investors takes the verb will take part in
-- no verb exists for the subject one of them
-- no adjective exists for the subject one of them
-- we have a stranded subject

• 99 percent of the time, in the "___ of ____" structure, the second word that refers to the group is whom or which
ELIMINATE B


• as in (B), the subject one of them who is Russian lacks a verb
• and does not help. If anything, the word and implies that this "one of them who is Russian" belongs to some vague "them" group and exists in addition to the group of investors already mentioned.
• not in ____ of _____ format
ELIMINATE C


• Bingo.
→ whom refers to the investors
→ OF lets us know that "one" is a part of that group
→ one refers to the singular part of that group; that person gets highlighted because she is Russian
KEEP



• which is not used for human beings.
If needed, compare D to E in order to remind yourself: Which and that refer to things, whereas who and whom refer to people.
ELIMINATE E

Notes

→ subgroup modifiers revisited

Subgroup modifiers simply spotlight part of a larger group.

Subgroup modifiers are formed with

(1) a SAMAN "part" word + OF + relative pronoun (SANAM = some, any, none, all, more/most)
most of whom, all of which, none of whom

(2) a "quantity" word + OF + relative pronoun ("quantity" = each, either, neither, many, few, both, one, etc.)
neither of which, many of whom, each of which

(3) a SAMAN or quantity word + OF + a "regular" pronoun such as them
all of them, both of them, neither of them

The formalistic guideline: Use SANAM/quantifier + OF + relative pronoun/pronoun

→ Examples of each combination
i) The children, all of whom like candy, are excited for Halloween. (SANAM + OF + relative pronoun)

ii) The rise of autocratically led countries, each of which was initiated by stoking fear and hatred, seems not to bother far too many Americans. (quantity word + OF+ relative pronoun)

iii) The manuscripts, a few of them deteriorating quickly, were sealed in a vault.
→ this kind of modification is fairly rare. Just recognize that a few of them highlights a portion of the whole group of manuscripts and that deteriorating quickly is an adjective that describes a few of the manuscripts.

→ Also recognize that if a regular pronoun is used, a verb is not used in the modifying phrase.

→ On the other hand, if a relative pronoun is used, a verb must be used in the modifying clause.

COMMENTS

UjjawalD , gkannampGVK , and TheGriffin , welcome to SC Butler.

gkannampGVK , you wrote

Yes, whom is always an object pronoun. No exceptions.

You are wondering how whom, an object pronoun, could be the subject of a relative clause, I think?

Nice catch. No inconsistency exists, though.
Whom isn't the subject.

Depending upon whom you believe, the subject of that clause is either
ONE (singular)
or
ONE OF WHOM, in which one is a "head noun" that controls the verb and is still singular
→ The word one, singular, in the phrase one of whom either is or anchors the subject.
→ in both cases, singular one controls the verb.

Maybe one of which, also a singular subject and similar to one of whom, will make this case clearer.
Correct: I saw a basket of apples, one of which was extra big.
Wrong: I saw a basket of apples, one of which were extra big.

You might want to read this post, here, about subgroup modifiers.

I am glad to see dialogue and always glad to have questions.

These answers range from good to excellent. Be safe, everyone.