Article 26 VCLT codifies the maxim:

1. Article 26 VCLT: 'Every treaty in force is binding upon the parties to it and must be performed by them in good faith.' 2. The Latin maxim PACTA SUNT SERVANDA ('agreements must be kept') is the foundational principle of treaty law. 3. Article 27 VCLT adds that a party may not invoke the provisions of its internal law as justification for its failure to perform a treaty. 4. This rule is itself a rule of customary international law applicable even to non-VCLT parties. 5. Hence option B is correct. _Source: UN Charter 1945 / Vienna Convention on Law of Treaties 1969 / ICJ Statute / ARSIWA 2001 — Vienna Convention on the Law of Treaties 1969, Article 26_