Final answer:
Achilles' argument to keep the body and do what he wills with it is based on his right as the victor in battle and the show of dominance and power in warfare.
Step-by-step explanation:
The argument to allow Achilles to keep the body and do what he wills with it is his right as the victor in battle (Option D). In ancient Greek society, the spoils of war were often awarded to the victor, including the bodies of fallen enemies. This was seen as a show of dominance and power in warfare (Option C). Additionally, there may have been an ancient tradition of desecrating enemies' bodies (Option B), but this is not explicitly mentioned in the provided information. There is no mention of Zeus's decree for the treatment of fallen enemies (Option A).