Answer
(d)
Work Step by Step
Atmospheric brown clouds result from a combination of primary and secondary pollutants, as well as the presence of $PM_{2.5}$ (particulate matter with a diameter of $2.5$ micrometers or smaller) and $O_3$ (ozone).
Primary pollutants are directly emitted into the atmosphere, while secondary pollutants are formed through chemical reactions involving primary pollutants.
The answer is (d).
