Which of these countries is not a United Nations member state?

Niue is not a United Nations member state because it is a self-governing territory in free association with New Zealand. While Niue has its own government and has control over its internal affairs, it relies on New Zealand for defense, foreign affairs, and citizenship. Since it does not possess full sovereignty, Niue is not eligible for full membership in the United Nations. However, it maintains a non-member state relationship with the UN, with limited participation in certain UN agencies and activities.