How did the Code Talkers keep their code secure?

The correct answer is that the Code Talkers kept their code secure by using their native language. This strategy involved translating military messages into a Native American language, such as Navajo, which was not understood by enemy forces. The uniqueness and complexity of the languages chosen made it virtually impossible for opponents to decipher the messages without prior knowledge of those languages.

Utilizing their native language meant that even if enemy intercepts were made, they would likely misinterpret or completely fail to understand the content, thus providing a significant security advantage. The Code Talkers not only ensured that critical information remained confidential but also played a crucial role in military operations during World War II, illustrating the effectiveness of using an indigenous language in a combat setting.

The other options do not convey as effective a method for maintaining the security of military communication. Writing everything down would potentially expose messages to interception, and complex ciphers, while useful, could still be cracked given enough time. Limiting communication to radio only may reduce the number of ways messages could be intercepted but does not inherently secure the content of those messages.