I believ the correct answer is B.) Ambition --- He is ready to quit trying.
Answer: Ambition
Explanation:
Keats´ poem reflects the thoughts he has about dying young and the legacy and experiences he wants for himself while still alive.
Love and fame, two things that most people, especially great poets, usually want, are described as "nothing" in the last few lines of the poem, showing that Keats seems to have given up on his ambitions.
By then, the worries he showed at the beginning of the poem, seem to have suddenly vanished.