This describes a way in which unrestricted free speech in fact discourages people from speaking. This makes the hypothetical chilling effect described by the letter-writer more concrete, so better justifies the conclusion that unfettered free speech actually restricts free speech.

This describes the potential characteristics of some freely-expressed views, but doesn’t indicate that free speech would be inhibited. For the conclusion, it doesn’t matter whether freely-expressed views are offensive or not.

This is irrelevant. The author claims that an effect of unrestricted free speech is the inhibition of free speech. What people should or shouldn’t do is irrelevant to whether the causal relationship posited by the author exists.

This describes how one might object to the author’s conclusion by way of analogy. We’re looking for an answer that justifies the author’s position, not one that attempts to debunk it, so this isn't useful.

This would weaken the author’s argument by undermining the idea that unrestricted free speech would actually silence people. The correct answer should describe how unrestricted speech makes others less likely to voice their views, not more likely.