Volcanoes in East Asia do pose several challenges, but the primary concern among the options you've provided is:
They sometimes threaten densely populated areas.
Many regions in East Asia, including countries like Japan and Indonesia, are home to large populations that live in proximity to active volcanoes. Eruptions can lead to significant destruction, loss of life, and disruption of daily life, making this a critical issue in those areas.
While the other options might have some relevance, the immediate risk to human life and infrastructure from volcanic eruptions is the most pressing challenge.