Diplomacy was a necessity for the Byzantine Empire because it had enemies on all of its borders and there was always a threat of invasion while for a certain time it had limited military power. The military force of the Byzantine Empire never exceeded the number of 140.000. While the threat of invasion came from all quarters, e.g., from Nomadic people to Germanic people, from Slavs to Hungarians and Russians, its internal resources were not enough to keep a military permanently in order to give a military response to these threats. Since the rulers of the empire were aware of their military weakness, they first adapted the practices of former civilizations and developed their own way of diplomacy with their political and cultural contributions. The correct answer is D.