What forms the sources of Customary International Law?

Customary International Law is fundamentally derived from the consistent and general practices of states that are accepted as legally binding. This concept highlights that for a practice to evolve into customary law, it must not only be a frequently observed behavior among states but must also be accompanied by a belief that such behavior is required by law, known as "opinio juris."

Therefore, customs and practices that are recognized by a significant number of states, and which imply a legal obligation, form the core of Customary International Law. This includes various norms that have developed over time, such as the prohibition against torture or the principle of diplomatic immunity, which states observe and acknowledge as part of their legal responsibilities.

The other options, while they have relevance in the broader context of international law, do not accurately encapsulate the essence of Customary International Law. Legislation from multiple countries pertains more to national law or harmonization efforts rather than customary practice. Written documents by international organizations could take the form of guidelines or recommendations but do not establish customary law on their own. Similarly, international treaties may contribute to shaping norms and practices but are distinct from customary law, which exists independently of written agreements.