Option 1: Cambodia - This is a member of ASEAN. ASEAN stands for the Association of Southeast Asian Nations, and Cambodia is one of the ten member countries.
Option 2: Laos - This is also a member of ASEAN. Laos is one of the ten countries that make up ASEAN.
Option 3: Singapore - This is a member of ASEAN. Singapore is one of the founding members of ASEAN and plays an active role in the organization.
Option 4: South Korea - This is not a member of ASEAN. South Korea is not part of ASEAN. It is a country located in East Asia, but it is not included in the Association of Southeast Asian Nations.
Therefore, the correct answer is option 4. South Korea is not a member of ASEAN.