How long did Franklin D. Roosevelt serve as U.S. president?

Franklin D. Roosevelt served as the 32nd president of the United States for a total of 12 years, which makes that choice the correct answer. He was elected to four terms beginning in March 1933 and served until his death in April 1945. This duration is notable because no other president had served more than two terms before him, which eventually led to the 22nd Amendment to the Constitution, limiting future presidents to two terms. His presidency spanned significant historical events, including the Great Depression and World War II, underscoring the importance of his long and impactful tenure in American history.