During the Second Spanish Period, which began in 1783 and lasted until 1821, the correct answer is:
A. Pinckney's treaty established the northern boundary of Florida.
Pinckney's Treaty (also known as the Treaty of San Lorenzo), signed in 1795, defined the borders between Spanish Florida and the United States and allowed for the navigation of the Mississippi River.
Option B is incorrect because significant battles of the Revolutionary War primarily occurred before the Second Spanish Period (1775-1783).
Option C is incorrect as the Treaty of Paris (1763) actually transferred Florida from Spain to Great Britain, ending the First Spanish Period.
Option D is more applicable to the period following the Revolutionary War and does not specifically define occurrences unique to the Second Spanish Period.