In the story "The Battle Picnic," when Manfred states that he is battling on the "proper side," the word "proper" suggests that:
D. Manfred believes his side is supported by the forces of justice.
This choice implies that Manfred thinks his cause is just and morally right. By using the word "proper," it indicates that Manfred perceives his side as being on the side of righteousness or what is right in the context of the battle. It reflects a sense of moral superiority or legitimacy in his stance.