Explanation:

Brunei Darussalam served as the Chair of ASEAN for the year 2021. Consequently, the 38th and 39th ASEAN Summits and related summits, which took place from October 26 to 28, 2021, were hosted by Brunei. While the meetings were held virtually due to the global pandemic, Brunei was the designated host nation. Options (2) Vietnam, (3) Thailand, and (4) Singapore were not the chairs of ASEAN in 2021 and thus did not host the main summits for that year. Brunei's chairmanship signifies its role as the host.