Final answer:
Option (B), John Quincy Adams negotiated the Adams-Onís Treaty, resulting in the U.S. acquiring Florida and setting the foundation for further expansion.
Step-by-step explanation:
As Secretary of State, John Quincy Adams's role was notably marked by the negotiation of the Adams-Onís Treaty in 1819, in which the United States obtained control of Florida. Adams skillfully leveraged the situation created by Andrew Jackson's incursion into Spanish Florida to pressure Spain to cede the territory.
This diplomatic achievement not only brought Florida under American control but also involved Spain relinquishing its claims to the Oregon Country, while the U.S. temporarily renounced its claims to Texas, thereby setting the stage for further continental expansion.