Article 51 UN Charter preserves the inherent right of:

1. Article 51 UN Charter: 'Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.' 2. Key requirements: (i) ARMED ATTACK; (ii) immediate; (iii) report to UNSC. 3. The ICJ in Nicaragua v. USA, 1986 ICJ Rep 14 elaborated the requirements (proportionality, necessity). 4. Pre-emptive self-defence is controversial; 'anticipatory' self-defence is invoked in some contexts. 5. Hence option B is correct. _Source: UN Charter 1945 / Vienna Convention on Law of Treaties 1969 / ICJ Statute / ARSIWA 2001 — UN Charter 1945, Article 51_