I'm not sure if there is a specific line I would point to, but I can try to talk through how I would have gotten to D.
The point of bringing up minor eruptions is to show that they do not have an impact on the climate, and that major ones do have an impact, albeit not as large of an impact as previously thought.
The way that volcanoes impact the climate is by spewing debris into the atmosphere, and cooling it as a result (line 4-6ish).
Since a minor eruption is probably one that is less intense or severe than a major eruption, I expect that answer will have something to do with the actual intensity of the eruption itself, which leaves A and D, and since we care about debris, and not the amount of lava, I would go with D.