The question presents three pairs each containing an organization and its corresponding location.

For the first pair, Asian Development Bank indeed operates in Asia, but it is headquartered in Mandaluyong, Philippines, not Tokyo. Hence, the first pair is incorrect.

The second pair, Asia-Pacific Economic Cooperation (APEC), is a regional economic forum, but it doesn`t have a specific headquarters as it operates in 21 member economies across the Asia-Pacific region. The Secretariat of APEC, however, is located in Singapore, which makes this pair partially correct.

The third pair, Association of South East Asian Nations (ASEAN), is a regional intergovernmental organization. Its Secretariat is located in Jakarta, Indonesia, not Bangkok. Thus, the third pair is incorrect.

Based on the above, it is clear that only the second pair is partially correct, which corresponds with option 2, `2 only`. So, the given answer is correct.