Answer:
I would say that the excerpt from this narrative that most clearly shows the reason why the author wrote it is the first one: "The most important thing I learned that day, though, was that one person can have a big impact on others, even without realizing it."
Step-by-step explanation:
I chose this option because here the speaker realizes how important it can be to have a model, and I think the whole idea for the writer to do it is to create impact in others. The speaker understands how important it is to show and share good habits, kindness, etc. He knows that, by giving the example, one's life can have a great impact on others and that it can be contagious. He feels he can do good by sharing an experience like the one he is choosing to tell here.