The doctor here had earned enough to live a comfortable life. The the sentence expresses here that the doctor was not actually prosperous enough. Prosperity rather not the option as expressed by the word "enough" which marks the abundance just enough not quiet enough.
What does comfortable mean?
Comfortable here refers to living a life that has an ease of privilege but not enough to person to become affluent in the society.
What does prosperity mean?
Prosperity here refers to the abundance, the person here has everything possible to make himself content and happy through material things.
The sentence here exclaims the material aspect of doctors earnings not the mental peace of providing medical services. The doctor hence clearly has no means earning through vicious means.
Hence, option A is correct.
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