Final answer:
The agreement between France and President Thomas Jefferson, known as the Louisiana Purchase, resulted in the people in the area becoming citizens of the United States.
Step-by-step explanation:
The agreement between France and President Thomas Jefferson, known as the Louisiana Purchase, resulted in significant changes for the people in the area. The correct answer to the question is option A: They were now citizens of the United States. With this agreement, the United States gained control over a vast territory, effectively doubling its size. The people in the area of the Louisiana Purchase became citizens of the United States and enjoyed the rights and privileges associated with that.