Which of the following is a secondary air pollutant?
(A) Ozone
(B) Carbon dioxide
(C) carbon monoxide
(D) Surfer dioxide

Hint: Air Pollutants can be solid particles, liquid droplets, or gases. They can be natural or manmade. In this case it can be both helpful and harmful depending on where it is present. It blocks the harmful UV rays coming from the Sun.

Complete step by step solution:
Primary pollutants are the air pollutants that are emitted directly from a source in a harmful form. Some are released by natural processes, like ash from volcanoes while most of them are released by human activities.
Secondary pollutants are the air pollutants that are not directly emitted. They are formed when other pollutants (primary pollutants) react in the atmosphere and make them harmful.
Now, let's take a look at our options:
A. Ozone
Tropospheric Ozone is majorly formed when various elements such as volatile organic compounds, CO, \[N{{O}_{x}}\], etc. react in the presence of sunlight. Unlike stratospheric ozone, which protects the planet from the ultraviolet radiation of the sun, this is harmful. So, it is a secondary pollutant.

B. Carbon dioxide
Carbon dioxide (\[C{{O}_{2}}\]) is a greenhouse gas which is emitted directly to the atmosphere in the form of combustion, respiration, etc. So, it is a primary pollutant.

C. Carbon monoxide
Carbon monoxide (CO) is formed as a result of the incomplete combustion of organic matter and released directly in the atmosphere. It is a toxic gas, even in small quantities. So, it is a primary pollutant.

D. Sulphur dioxide
Sulfur dioxide (\[S{{O}_{2}}\]) which is released in the atmosphere is mainly a consequence of human activities such as the burning of coal or oil. Its main source of acid rain, when it transforms into sulfuric acid (\[{{H}_{2}}S{{O}_{4}}\]). So, it is a primary pollutant.

So, the correct option is (a).

Note: Ozone present in the atmosphere is very dangerous for human health as it can cause respiratory problems or eye irritation. It also affects the environment adversely by slowing the process of photosynthesis.