Final answer:
Marquis de Lafayette was the young French officer who became a general on Washington's staff and his lifelong friend.
Step-by-step explanation:
The young French officer who became a general on Washington's staff and his lifelong friend is Marquis de Lafayette. Lafayette played a significant role in the American Revolution, providing military expertise and leadership. He formed close bonds with Washington and remained a friend and supporter of the United States throughout his life.
Learn more about Marquis de Lafayette's role in the American Revolution