The idea behind the Personal Long Letter campaign is that a single imp

Hey All,

Lots of ideas on this one, but no one's taken it apart yet. That's what I'm here for!

The idea behind the Personal Long Letter campaign is that a single impassioned constituent may sway a lawmaker’s opinion, whereas a half-dozen banded together only causes him alarm.

This is clearly a verb tense and subject-verb agreement question. We notice the former in the split between "causes" and "has been caused". We notice the latter int he split between "causes" and "cause" (or "has" and "have"). For subject-verb agreement, we check the subject and make sure it matches. "Half-dozen (constituents)" is plural, so we need "cause" or "have". As for verb tense, you always check it against some context verb in the sentence. In this case, we have "is", a present tense verb. Then we ask ourselves, is there any good reason to change tense? In this case, there is not.

 a half-dozen banded together only causes him alarm
Problem: "Causes" doesn't match the subject "half dozen (constituents)"

 only alarm is caused by a half-dozen banded together
PROBLEM: This is actually a concision issue. No reason to switch to the passive voice here. While this is a rule, it's VERY rare on the real test, and comes up way more when people are trying to build questions. DO NOT cross something off just because it's passive. We're only doing it here because there's a perfectly great answer WITHOUT the passive voice elsewhere.

 only alarm has been caused by a half-dozen banded together
PROBLEM: No reason to switch to the present perfect tense ("has been caused").

 a half-dozen banded together only cause him alarm
ANSWER: Correct tense, correct subject-verb agreement.

 a half-dozen have caused him only alarm when banded together
PROBLEM: No reason to switch to the present perfect tense ("have caused").

Hope that helps!

-t