One of the important functions of the United Nations is to decide if

In the original sentence, the plural pronoun “they” has no clear antecedent. It is trying
to refer to the United Nations, which is a singular noun. Additionally, on the GMAT, “if”
is used for a conditional idea, while “whether” is used for an alternative or possibility.
In this sentence, the United Nations is choosing between two possibilities: either it will
recognize the legitimacy of a new government or it will not. Thus, “whether” is the
appropriate word.

(A) This choice is incorrect as it repeats the original sentence.

(B) CORRECT. This choice correctly resolves the pronoun issue, and is the only
answer choice that avoids serious problems.

(C) The use of “deciding” as opposed to the infinitive “decide” is appropriate, but this
sentence incorrectly separates the modifier “that assumed power through violence”
from the modified noun “government,” making this choice awkward and incorrect.

(D) In this choice, “if” is incorrectly used to distinguish between alternatives.
Furthermore the non-restrictive relative pronoun “which” is used to introduce a
restrictive clause. Because the clause “that assumed power through violence” is
essential to the meaning of the sentence, “that” must introduce the clause.
Conversely, “which” is used to introduce nonessential elements.

(E) In this choice, the pronoun “they” has no clear antecedent, since the United
Nations is a singular entity.