Final answer:
The quote 'C. began now seriously to reflect upon what I had done' shows a change in the narrator's mindset, indicating a moment of self-reflection and possible regret.
Step-by-step explanation:
The quotation that shows the narrator changed his mind is 'C. began now seriously to reflect upon what I had done'. This statement indicates a shift in the narrator's thought process, suggesting that they are considering the consequences of their actions and may be experiencing regret or a desire to make a change. This change is further supported by the examples provided which depict moments of reflection, realizations, and transformations in various characters' perspectives.