Answer:
This source adds to the reader's understanding of Lincoln in the sense that:
B. It shows that he took his leadership roles seriously.
Step-by-step explanation:
According to the passage, one of the reasons why Lincoln considered his time in the war successful was the fact that he was elected captain by the men in his company. That already shows Lincoln values his role as a leader. But the passage also makes a point of mentioning that he was "very proud of, yet humbled by, their confidence in him." This information lets us know that he was not an overly confident men. Maybe he wouldn't have volunteered himself to become a captain but, since the men trusted him, he was happy to do it. With that in mind, we can choose letter B as the best option:
B. It shows that he took his leadership roles seriously.