The correct response is:
The historical context allows the reader to understand that the “Captain” is Abraham Lincoln.
Knowing the historical context of the poem reveals that Whitman wrote "O Captain! My Captain!" in response to the assassination of Abraham Lincoln, where Lincoln is metaphorically depicted as the captain of the ship (the nation) that has successfully weathered the turbulent times of the Civil War. This understanding significantly enriches the reader's interpretation of the stanza, particularly regarding themes of loss, leadership, and the culmination of struggle.