Answer:
B. He was losing a friend.
Step-by-step explanation:
The passage talks about how Socrates cheerfully accepted his death by drinking the goblet of poison, and Plato (the narrator "I" in here) began to weep. Yet, it's not because of his sadness for Socrates' life, as we might think. The given lines indicate something else:
"It was not for him that I wept, but for my own misfortune in being deprived of such a friend"
"deprived of such a friend" essentially just means losing a friend, so Plato wept because he lost a good friend.
The answer is B.