The correct answer is - d. ash.
Volcanoes let out on the surface and in the air lots of different things, from lava and huge rocks to ashes and toxic gases. The one thing that causes the biggest trouble in air pollution is the ash. When a volcano erupts it lets out huge amounts of ash that can cover up whole regions, and they can have an impact of the global climate by covering the sky with ashes that stop the sun rays reach the Earth. These ashes are a very big pollutant of the air and completely change its structure, for the living creatures air with big amount of ash in it can be lethal.