What did the Japanese military do regarding the use of code during the Pacific War?

The correct choice highlights that the Japanese military made efforts to develop their own codes during the Pacific War but struggled to create ones that could rival the effectiveness of those used by the United States, including the Navajo code. Although Japan had its own coding systems, they often lacked the effectiveness and efficiency seen in the American code, particularly the Navajo Code Talkers' language-based codes which were unbreakable due to their uniqueness and complexity.

The Japanese military recognized the necessity of coding communications to secure their strategies and movements from enemy interception. However, their attempts often fell short, particularly in establishing codes that were both secure and practical for rapid communication on the battlefield. This contrast emphasizes the success of the U.S. in utilizing a code that was not only effective but also relatively simple to use due to the linguistic skills of the Navajo speakers involved.

The other choices either inaccurately depict the Japanese action towards coding or underestimate the capabilities of their code development, which ultimately did not measure up to the effectiveness of American codes.