In the past few months, there has been extensive dispute over if fare

KAPLAN OFFICIAL EXPLANATION:

The key idiom in the sentence is dispute; the sentence asks you to determine which is the correct preposition that should follow it. Skimming through the answer choices, you see that you have several options. Excerpt the idiom and ask yourself, assuming that one has to discuss such things, does one talk about a dispute "over" an issue, "about" an issue, or "concerning" an issue? The first is the correct idiom, so eliminate (В), (C), and (E). The only difference between (A) and (D) is the use of if or whether. Memorize this fact if you don't know it already: whether is the correct choice when the sentence describes alternatives; if is correct when it describes a hypothetical situation (whether to participate versus if he participated, he would...). Since this sentence presents two alternative options (first or last recourse), whether is correct. That leaves (D).

An 800 test taker knows that idioms can't be explained by stable rules; idioms are what they are and there's often no more to say about them. He identifies the idioms he doesn't know and memorizes them, knowing that idioms will show up frequently on the exam.