According to Bull, an international system has three defining characteristics:
1. First, it requires the presence of multiple sovereign states.
2. Second, regular relations and interactions of these states must exist as a part of a system.
3. Third, these states must act while being bound by shared rules and norms that bring them together.
(Bull, 1995: 225)
From this assessment, we can conclude that the Peace of Westphalia in 1648 constituted the starting point of the international system: Europe adopted a specific kind of sovereignty and regular interactions among mutually-recognized sovereign states emerged. The correct answer is D.