Public International Law regulates:

The conduct of one state with another

The correct answer is Option (1) → The conduct of one state with another

Public International Law is the law that regulates relations between states. Public International law is different from other types of laws because it is concerned with interstate regulation, i.e. it deals in regulating the conduct of one state with another and is not concerned with the relations between private entities (legal and natural persons) and even the domestic laws of any country. The primary objective of Public International law is to provide for a framework of rules and regulations which help in fostering stable and organized international relations.