Yesss???? What do you mean? :(
The adverb " very " modifies the adjective " useful ".
It's not a. obviously because he doesn't know that she and her father fought over her not wanting to marry him. It's not b. because he acts like she is already his wife and I quote says "Till then, adieu, and keep this holy kiss." It isn't c. because the whole reason her father insisted he marry her so quickly was because of her deep grieving over "Tybalt", which concludes that the answer is d. (Sorry just wanted to be thorough so you wouldn't be confused.)
