Who was the first president to speak at the National Convention?

The correct choice is Dwight D. Eisenhower because he was the first sitting U.S. president to address the National FFA Convention, doing so in 1953. Eisenhower's participation marked a significant moment for the organization, highlighting the importance of agricultural education and leadership at the national level. His address reflected the values of FFA and the agricultural community, and it helped to elevate the profile of the organization in the eyes of the public and policymakers.

The other options, while notable figures in American history, did not speak at the National FFA Convention during their presidencies. Theodore Roosevelt and Harry S. Truman served in a period prior to the FFA's formal recognition and establishment, while John F. Kennedy did speak at the convention, but he was not the first president to do so. Understanding Eisenhower's role as the initial presidential speaker underlines the engagement between national leadership and agricultural education during that era.