Final answer:
Brinker Hadley's key conflict in 'A Separate Peace' is his tension with Gene over the circumstances surrounding Phineas's accident.
Step-by-step explanation:
One of the key conflicts that Brinker Hadley encounters in "A Separate Peace" is C. His tension with Gene regarding Phineas's accident. Brinker is a prominent character in the novel who is known for his leadership qualities and his role as a key member of the student judiciary at Devon. His conflict with Gene comes to a head when he organizes a mock trial to uncover the truth behind Phineas's fall, which ultimately results in increasing tension between the characters, bringing the underlying issues into the open.