Which group was primarily responsible for creating the codes used by Code Talkers during World War II?

The group primarily responsible for creating the codes used by Code Talkers during World War II was the Marine Corps. This choice is correct because the Code Talkers, most famously the Navajo Code Talkers, were recruited specifically by the Marine Corps to develop an unbreakable code based on the Navajo language. This code was crucial in the Pacific Theater for secure communication, allowing Marines to convey sensitive information without fear of interception by enemy forces.

The Marine Corps recognized the unique qualities of the Navajo language, including its complexity and tonal nuances, which made it particularly effective for military coding purposes. The code created was never written down, relying instead on oral transmission, further enhancing its security.

In contrast, while the Army, Navy, and Air Force may have utilized various forms of coded communication, it was specifically the Marine Corps that initiated and implemented the use of the Navajo Code Talkers' system, highlighting their integral role in the success of operations during the war.