Final answer:
The Navajo code was unique because it was a military code based on the Navajo language, making it extremely difficult for the enemy to decode.
Step-by-step explanation:
The correct answer is option D. The Navajo code, also known as the Navajo Code Talkers code, was a military code used by the United States Marines during World War II. It was a code based on the Navajo language, which was almost impossible for the enemies to decode due to its complexity and the fact that it had no written version at the time. This code was a critical asset for the U.S. military as it helped protect sensitive information and communications from being intercepted and understood by the enemy.
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