Final answer:
Henry A. Wallace was replaced by Harry S. Truman as FDR's Vice President in the 1944 election to appeal to moderate voters and due to Roosevelt's declining health.
Step-by-step explanation:
The Vice President who was replaced by Harry S. Truman as FDR's (Franklin D. Roosevelt's) running mate was Henry A. Wallace.
In the 1944 election, FDR chose to replace Wallace with Truman to balance the ticket and attract more moderate voters, amidst concerns over Roosevelt's declining health and a shift towards more conservative politics in the United States.
As a result, Truman became the Vice President and later assumed the presidency following FDR's death on April 12, 1945, after Roosevelt suffered a massive stroke.