Final answer:
A timeline depicting the major milestones of Theodore Roosevelt would be the most effective graphic to support readers' understanding of his life in an essay, as it provides a clear, chronological visualization of his influential career and personal development.
Step-by-step explanation:
Leon is writing an essay about the life of Theodore Roosevelt for his history class. Among the options provided, the graphic that would most likely support readers' understanding of the topic is B. A timeline depicting the major milestones of Theodore Roosevelt. This type of visual aid would effectively communicate the significant events throughout Roosevelt's life and career, aligning closely with the subject of the essay. This enables the reader to follow Roosevelt's progression from a sickly child to a robust leader, as well as his impact on America's development into a progressive nation. Presenting this information in a chronological format would help illustrate how Roosevelt's actions and policies evolved over time, reflecting his growth as a leader and his influence on the country.