The answer choice you're referring to (C) can be simplified to say:

OPEC Officials announced that the group will pare production next year only if the output of non-OPEC nations would be trimmed.

There is no need to use "would" in front of "be trimmed" but the issue is less redundancy than improper use of the subjunctive mood.

The subjunctive mood is used in "If [condition] then [consequence]" statements. Often, the "then" is implied. For instance, "If you do that again, [then] I will be upset with you". In some cases, the "then" clause even comes up first: "I will succeed if I persevere" is equivalent to "If I persevere then I will succeed".

In the subjunctive mood, "would" is sometimes used in the "then" clause: "If I were in your position, [then] I would do the same". The "would" should not go inside the "if" clause.

wrong: The GMAT will become more affordable if its fee would be decreased.
right: The GMAT will become more affordable if its fee is decreased.

In the sentence in question you mentioned:
wrong: "the group will pare production if output would be trimmed"
right: "the group will pare production if output is trimmed"
(an even better sentence will avoid passive altogether and use "if nations trim output")

A full discussion of this question, and of all OG questions since the OG10, can be found in the GMATFix App. The solution below is taken from the App.



-Patrick