Final answer:
Thomas Jefferson ceased his opposition to the expansion of the navy when he saw the effectiveness of agile coastal naval crafts in defeating the Barbary Coast pirates during the Tripolitan War.
Step-by-step explanation:
Thomas Jefferson ceased his opposition to the expansion of the navy when the strategic usefulness of employing significant numbers of agile coastal naval crafts became apparent in the U.S. Navy's defeat of the Barbary Coast pirates during the Tripolitan War (Option A).
Despite his earlier preference for a smaller navy and reliance on militia forces, the challenges faced during the Tripolitan War showed Jefferson the importance of a capable naval force in protecting American interests. Jefferson's administration is marked by tensions with European powers and the need for America to assert its neutrality and protect its merchants. As the Napoleonic Wars escalated, both France and Britain regularly infringed upon American shipping, which highlighted the need for naval power to defend against such transgressions.
The invasion and harassment of American shipping intensified Jefferson's shift from his initial opposition. The success against the Barbary pirates reiterated the need for naval strength even more forcefully, showing the potential of a well-deployed maritime force in deterring threats and safeguarding American commerce.