Option A: 'Reward' means 'a fair return for one's hard work'.
It agrees to the given sentence: effort only fully releases its returns after a person refuses to quit, and works hard to achieve their goal.
Hence option A is correct.
Option B: 'Result' is the favourable outcome of one's work. The word isn't as suitable as 'reward', because 'result' refers to any outcome, as opposed to the gift or reward that one achieves due to hard work. The given sentence focuses on the reward, rather than result.
Option C: 'Rebuke' means 'criticizing someone heavily for something'.
Option D: 'Retort' means 'to respond in an angry or witty manner'.
These words do not seem relevant to this sentence.
Hence options B, C and D are incorrect.