Final answer:
The international norm against assassination of political leaders has become more restrictive over the centuries, moving from a time when such actions were more common within political power struggles, to the current era where global consensus and international law condemn such acts. So the correct option is C.
Step-by-step explanation:
The international norm against assassination of political leaders has evolved considerably since ancient Roman times. Initially, political assassinations might have seemed like 'business as usual' during certain periods, such as the aftermath of the Punic Wars and the decay of the Roman Empire, where political power struggles and assassinations by Roman elites were not uncommon. However, over the centuries, especially with the rise of international law and global institutions such as the United Nations, the norm has become far more restrictive. Acts of assassination are generally condemned, and state leaders are often compelled to justify their actions within the framework of accepted international norms, signifying a higher degree of global consensus against such practices.
While enforcement mechanisms for international law can be limited, the compliance with these norms is surprisingly high, indicating a trend towards resolving conflicts through diplomacy and peace rather than violent actions like assassinations. The change in normative behavior regarding assassinations echoes the broader evolution of international relations and political behavior, moving towards more predictable and rule-based systems.