The TWO-element approach to customary international law (state practice + opinio juris) was famously stated in:

1. The two-element approach to customary international law is the orthodox view: (1) general state practice; (2) opinio juris. 2. The ICJ in North Sea Continental Shelf Cases (1969) articulated the test most clearly, holding that the practice must be both extensive and virtually uniform AND accompanied by opinio juris. 3. The ILC's Draft Conclusions on Identification of Customary International Law (2018) confirm this approach in Conclusion 2. 4. Some 'modern custom' theorists (Bin Cheng, Roberts) have argued for a more flexible reading. 5. Hence option B is correct. _Source: UN Charter 1945 / Vienna Convention on Law of Treaties 1969 / Geneva Conventions 1949 / ICJ Statute / UDHR ICCPR ICESCR — North Sea Continental Shelf Cases, 1969 ICJ Rep 3, paras. 71-77; ILC Conclusions on CIL 2018_