We start with the Peace of Westphalia that ended the Thirty Years’ War in 1648. It is from this peace settlement that modern international politics derived its fundamental characteristics:  the rise of the territorially-conceived sovereign state and the beginning of the state-system in Europe.  Furthermore, it paved the way for the development of crucial instruments such as diplomacy, international congresses, and international law that still constitute the core of International Relations.  As it can be understood from the information given, the correct answer is D. The international system itself as well as its principles, instruments, and political units have been undergoing constant change since the Peace of Westphalia. In other words, the nature of the international system’s principles, instruments and political units is subject to changes.