Option B is the right answer that the Navajo sent messages to prove that the code worked.
Navajo code talkers in the 20th century were the people who used the coded language as the method of their communication during wartime. Using their Native language, the Navajo Code Talkers developed a confidential military code which gave assistance to the United States to achieve success in the Pacific during World War II.
Navajo code talkers tested their code by sending messages so as to ensure that the code can be decoded by the receiver or not.