Volcanoes in East Asia pose several challenges, but the most prominent among the options you provided is that they sometimes threaten densely populated areas. Many countries in East Asia, such as Japan, Indonesia, and the Philippines, have large populations living in close proximity to active volcanoes. Eruptions can lead to catastrophic consequences including loss of life, destruction of infrastructure, and displacement of communities.
While volcanic activity can also affect farming stability and produce air pollutants, the immediate threat to human life and settlements from eruptions is a significant concern in a densely populated region like East Asia.