Wildfires are
a. a human source of air pollution
b. a natural source of air pollution
c. both a natural and human source of air pollution
d. none of the above

The answer is letter c.  It can be both natural and human source of pollution.  There are times when the intense heat can trigger a brush fire and can expand depending on how strong the wind blows.  It can be caused by humans.  it could due to carelessness of people leaving flammable materials unchecked. Or using too much fuel that when lit would cause a brush fire that would expand into a wildfires.  The smoke from it pollutes the air.

The correct option is C

A forest fire is the fire that spreads uncontrollably in forest or wild land, affecting vegetal fuels, flora and fauna. The beginning of the fire can be produced by natural causes or mainly by human action.

Natural forest fires have always been a normal element in the functioning of ecosystems. Fire has allowed the regeneration of diverse ecosystems and the production of a series of habitats in which different organisms can thrive. However, the enormous proliferation of fires caused by human activity in recent decades exceeds the capacity for natural recovery. The forest fires cause a great contamination in the air as well as Pollution of rivers that receive the rain waters that cross the burned zone dragging particles and ashes in suspension.

c. in the address bar