What was the last time the Fleet Admiral rank was used?

The last time the Fleet Admiral rank was used was during World War II. This rank was created during the war to recognize the highest level of command in the U.S. Navy, and it was awarded to only four individuals: William D. Leahy, Ernest King, Chester W. Nimitz, and William Halsey Jr. After World War II, the rank has not been utilized again, making its use during this period quite significant in naval history. The establishment of the rank was a direct response to the need for high-level strategic leadership to coordinate a large-scale naval force during the global conflict. Thus, selecting World War II as the correct answer accurately reflects the historical context and significance of the rank.