Final answer:
The ASEAN+3 forum includes member states of ASEAN plus China, Japan, and South Korea; therefore, the answer is South Korea.
Step-by-step explanation:
The ASEAN+3 forum is an organizational framework for cooperation that includes the ASEAN member states plus three additional countries: China, Japan, and South Korea. The correct answer to the student's question is option 1) South Korea.
ASEAN, which stands for the Association of Southeast Asian Nations, has been integral in promoting cooperation and integration among its members since the 1960s, eventually advocating for EU-style integration. ASEAN+3 is considered a key platform for discussing and managing regional challenges, including economic and financial collaboration among these East Asian nations.