Final answer:
The quote in the question exemplifies the concept of resilience or grit, which represents the persistence of a person in reaching a goal despite challenges like feeling discouraged or disgusted(option D).
Step-by-step explanation:
The quote '"... he never gets so discouraged or disgusted that he gives up doing it over again, because he knows very well it is important and worth the doing."' reflects a theme of resilience or grit. This idea is based on the premise that a person will continue striving towards a goal despite feeling discouraged or disgusted, because of the underlying belief in the importance and value of the goal. Grit, in this context, is not just a refusal to give up, but a driving force that propels an individual to persist in their efforts until they achieve success.
Hence, the answer is option D.