GMAT Sentence Correction (SC) | John first fell in love when he saw a woman sitting on a bench in the

I think E is definitely correct.

Reletive pronoun modifier (1) - "whose long hair shimmered in the sun" is encapsulated between 2 commas and provides additional info about "woman".

Verb-ing modifier (2) - "sitting on a bench in the park" modifies the noun "woman". Comma before "sitting" is a part of modifier 1.

So, the sentence as a whole is structured as : John first fell in love when he saw a woman(,whose long hair shimmered in the sun,)(sitting