Final answer:
Jefferson stopped opposing the navy expansion after recognizing its strategic importance following the victory over the Barbary Coast pirates. Therefore, the correct option is B.
Step-by-step explanation:
Thomas Jefferson ceased his opposition to the expansion of the navy when the navy's defeat of the Barbary Coast pirates during the Tripolitian War demonstrated the strategic usefulness of maintaining a strong naval force. Initially, Jefferson preferred to limit military engagements and maintain a small navy to avoid entangling alliances and excessive government spending. However, the continued harassment of American shipping by European powers such as Great Britain and France, as well as the challenges posed by the Barbary Pirates, highlighted the importance of a capable navy for protecting national interests and commerce.