The relationship between Gene and Finny in A Separate Peace is rife for metaphorical interpretation. Remember that both boys are studying and living in a relatively safe environment at Devon school, but soon, they'll be departing that safe haven to participate in some capacity in WWII, where boys slightly older than them are fighting and dying in great numbers. So, in the context of the book, the characters are caught between the childhood innocence and relative safety of the past and the danger, consequence, and responsibility of the future. Like other coming-of-age stories (or "bildungsroman"), A Separate Peace suggests that childhood innocence (and arguably any form of innocence) cannot survive in the face of the kind of knowledge and wisdom gained through adult experience.
If you translate this metaphor into the relationship between Finny and Gene in the novel, Finny represents the carefree, playful, idealistic nature of childhood, while Gene represents the neurotic, guilty, and worrisome qualities of a boy being sent into war to become a man. Some literary critics suggest that, while they are two separate characters in the novel, Finny and Gene represent two metaphorical parts of a whole person. When Finny dies at the end, it's like Gene's last bits of childhood innocence have died away too, leaving him an adult who can no longer ignore the dark, dangerous, and sad parts of human nature.
So, in the specific context of this question, with the choices provided, the answer is likely "B. Finny's death represents Gene's final passage into adulthood."