What was the main advantage of using Navajo as a code during military operations?

The main advantage of using Navajo as a code during military operations lies in its uniqueness, which provided a significant level of security against enemy interception and understanding. Unlike more commonly spoken languages, Navajo was not widely known or spoken outside of the Navajo Nation, making it an obscure choice for encryption. This obscurity meant that enemy forces were unlikely to have any insight into the language, thus allowing secure communication of critical military strategies and information.

The complexity and tonal nature of Navajo, along with specific terminology developed for military concepts, reinforced its effectiveness as a code. While the other options may highlight possible attributes of languages or codes, they do not capture the critical element of security derived from its uniqueness, which was key in allowing U.S. forces to communicate without the risk of enemy decoding their messages.