In which Article of the Code of Conduct does it say that one should not reveal military plans?

The correct answer is Article IV of the Code of Conduct. This article emphasizes the obligation of military personnel to keep military plans and information confidential, particularly in situations where they may be captured. It stresses the importance of safeguarding sensitive information that could jeopardize the security of operations or lives. The intent behind this is to maintain operational security and prevent the enemy from gaining any advantage through the disclosure of plans or strategies.

The other articles cover different aspects of conduct, such as the responsibilities to follow lawful orders and standards of behavior in combat and captivity. Article IV distinctly focuses on the importance of secrecy concerning military plans, which is why it is the correct answer in this context.