With Strengthen questions, we have to find the answer choice that will help make the conclusion 100% true/justified. The answer choices are extra premises that will either combine with the existing premises in the stimulus to bolster the conclusion (the correct one) or either do nothing or hurt the argument (incorrect ones). So to test them, we have to plug them into the stimulus and see which one actually strengthens the argument.
(A)--Sounds good at first read, but doesn't actually address the conclusion's scope of avoiding attacks on character, as nothing in the stimulus tells us that anything that reflects an inability to confront an argument means that thing should be avoided. This makes the answer choice irrelevant, and thus wrong.
(B)--Deals with audience reaction, which is clearly irrelevant. Immediate Loser.
(C)--This does a job similar to Justify or Supporter Assumption correct answers, by tying an existing premise to something that is unique to the conclusion. Here, that is tying the avoidance of a tactic (mentioned in the conclusion) with the not confronting an opponent's argument in the premise, making it the correct answer. To make this even clearer, we could conditionalize it:
This helps us get to where we want to end up in the conclusion (Avoid) and also ties us to what we already have in the premise (Relevant
(D)--Deals with the moral right of the person doing the character assassination, which is a reversal of the stimulus's premise. Immediate Loser.
(E)--A Mistaken Negation of (C); what we need is:
But this gives us:
Hope this clears things up!