occasionally, when it is completely unambiguous, "which" can refer to a whole NOUN PHRASE that immediately precedes the comma. in this case, this noun phrase is "X's letters to Y". (note that this noun phrase, as a unit, does immediately precede the comma.)
also, note the complete lack of grammatical ambiguity: "which" can't refer to dickinson, who is a person, and it's also followed by a plural verb. both of these pieces of evidence point to the noun phrase "X's letters to Y".
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here's the basic summary: if you have "X of Y, which..." then: * if Y works as the antecedent of "which", then "which" should stand for Y. * if Y doesn't work as the antecedent, but "X of Y" DOES work, then "which" can stand for "X of Y".
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i feel your pain, though. this problem is definitely a case of "learn surprising lessons from the correct answers, but never question the officially correct answers."