The correct answer to this open question is the following.
Although there are no options attached we can say the following.
Unfortunately, you did not attach the article, so we are only using our knowledge of social studies to answer this question.
Why might the Japanese military have been unable to break the Navajo Code Talkers Code?
Answer:
The Japanese military was not familiar with the language spoken by the Navajo Native American Indian tribe and there was no way they could learn the language as adults because it was so difficult and had no time to learn it in the middle of the war.
The code talkers developed the original version of the Navajo code in that they were looking for a special code to use in the war that was extremely difficult to be intercepted and translated by the enemy. That is why the code talkers developed a special code with elements of the Navajo language. During World War 1, Choctaw transmitted codified messages to the US Army headquarters in countries like France. Years later, by the 1940s during World War II, the US Army recruited Native American Indians such as Cherokees, Comanches, and Hopis. But in 1941, it was the Marine Corps the ones that hired Navajo Code Talkers to transmit secret communication during the war.