It's true that a noun modifier doesn't always have to physically touch the *word* that it is modifying, but only because sometimes it is modifying a noun that is expressed in more than one word. In OG #26, the subject is Emily Dickinson's letters to her sister. If we want to express the noun in one word, it would be "letters," but the reference to her sister is part of the noun phrase. Note that "which" cannot refer to her sister, who is a person.
The other examples here are more ambiguous. In the banana example, there is no reason that "which" might not refer to "cake," so I'd be cautious about using that. It would be better to say something along the lines of "The banana on the cake was over-ripened and completely ruined the cake." In the case of "A part of the work, which . . . ," I don't see a lot of workable continuations of that sentence. There is no need to use "which" here, unless we want to refer to "work" and not "part."
I hope this helps! Let me know if you have other examples or questions on this topic.
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Dmitry Farber | Manhattan GMAT Instructor | New York
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