Answer: Option (a) is the correct answer.
Step-by-step explanation:
The presence of harmful substances in air or environment is known as pollution.
For example, emission of smoke from automobiles releases harmful gases like carbon monoxide into the atmosphere leading to pollution.
Whereas cigarette smoke also causes pollution into the environment but out of the given options automobiles contribute greatly to air pollution.
Thus, we can conclude that the leading cause of air pollution is automobiles.