Final answer:
Thomas Jefferson reduced the size of the military to align with his beliefs in small government and agrarian society, to reduce national debt, and because he viewed a large standing military as a threat to citizens' freedom.
Step-by-step explanation:
Thomas Jefferson reduced the size of the country’s military primarily because he believed in a small government and an agrarian society, where a large standing military was deemed an unnecessarily expensive burden that could potentially threaten citizens' freedom. His actions reflected his vision that proper use of diplomacy could avoid international conflicts, thus making a standing army superfluous. Additionally, Jefferson was keen on reducing the nation's debt and viewed military expenses as a significant part of the federal budget that could be trimmed.
Jefferson's ideological stance against large government institutions led to his decision to reduce the professional military, preferring reliance on militias instead. Despite these reductions, Jefferson did see the need for professional officers, addressing this with the establishment of the United States Military Academy at West Point in 1802.
Therefore, the correct answer to why Thomas Jefferson reduced the size of the country's military is D. He worried that a large standing military was a threat to citizens' freedom.