Which code did the US Military employ during WWII that was impossible for the Japanese to break or understand?

The correct answer is the Navajo Code. During World War II, the United States utilized a unique code based on the Navajo language to facilitate secure communications, especially in the Pacific theater. The structure and complexity of the Navajo language made it exceptionally difficult for enemy forces, particularly the Japanese, to decipher. The use of Navajo speakers, known as "code talkers," ensured that messages could be transmitted quickly and accurately without the risk of interception by the enemy.

The Enigma Code, while significant in the European theater, was primarily used by Germany and related to their encryption systems, which were eventually broken by Allied codebreakers. The term "Secret Code" is vague and does not refer to a specific coding system used during the war, making it less relevant. Lastly, "Marine Code" does not specifically identify a unique code that was solely utilized or recognized in the context of World War II communications.