Which one of the following countries is not a member of ASEAN ?

The Association of Southeast Asian Nations (ASEAN) was established in 1967 by five founding members: Indonesia, Malaysia, the Philippines, Singapore, and Thailand [c1][t3]. Over the subsequent decades, the membership expanded to include Brunei Darussalam (1984), Vietnam (1995), Lao PDR and Myanmar (1997), and Cambodia (1999), bringing the total to ten member states [c1][t5]. While South Korea (Republic of Korea) maintains a significant relationship with the bloc as a dialogue partner and is part of the ASEAN Plus Three forum and the ASEAN-Korea Free Trade Area (AKFTA), it is not a member of ASEAN itself [c5][t1]. Indonesia, Thailand, and Vietnam are all confirmed members of the organization [c1][t4]. Recently, Timor-Leste was granted observer status with a path toward permanent membership, but South Korea remains an external partner [t7].

Sources

1. [1] Contemporary World Politics, Textbook in political science for Class XII (NCERT 2025 ed.) > Chapter 2: Contemporary Centres of Power > 20 Contemporary World Politics > p. 20
2. [2] https://ustr.gov/issue-areas/trade-organizations/association-southeast-asian-nations-asean
3. [3] Indian Economy, Vivek Singh (7th ed. 2023-24) > Chapter 13: International Organizations > Regional Comprehensive Economic Partnership (RCEP) > p. 394
4. [4] https://en.wikipedia.org/wiki/Member_states_of_ASEAN
5. [5] https://www.cfr.org/backgrounder/what-asean