To tackle the issue of Congressional campaign spending is
becoming embroiled in a war which is raging between those who support public financing with those who would lift the limits on the amount political parties and individuals may donate.
A. becoming embroiled in a war which is raging between those who support public financing with
B. becoming embroiled in a war raging among those who support public financing with
C. to become embroiled in a war raging between those who support public financing and
D. to become embroiled in a war which is raging among those who support public financing and
E. becoming embroiled in a war raging between those who support public financing and
Step 1: "To tackle" must be parallel with whatever begins the underlined portion, so "to become" is the correct option. Eliminate choice (A).
Step 2: (B) and (E) duplicate the error that occurs in the stem. Eliminate
Step 3: (C) and (D) remain. Choice (C) is uses the more concise "raging" to describe "war", whereas (D) uses "which is raging". (C) also uses "between" while (D) uses "among". Since we have two groups (those who support and those who would lift), "between" is the correct word. "Among" is correct when we're with more than two elements, groups, items.
To tackle... is sets up a parallel; to tackle is to become embroiled. (A), (B), and (E) don't create the parallel so eliminate them. The only difference between (C) and (D) is the use of between or among, respectively. Here's another rule to memorize: Between is the correct preposition when two options are presented, and among is correct when there are more than two. Since there are two sides in this dispute, between is correct. Eliminate (D) and (C) remains.