Introduction:
Secondary pollutants are formed when primary pollutants react with other substances in the atmosphere. These pollutants can have detrimental effects on human health, ecosystems, and the environment. In this question, we are asked to identify the pollutant that is not considered a secondary pollutant.

Explanation:
Let's analyze each option to determine whether it is a secondary pollutant or not.

Sulphuric Acid:
Sulphuric acid is formed when sulfur dioxide (SO2) reacts with oxygen (O2), water vapor (H2O), and other atmospheric compounds. Sulfur dioxide, emitted from burning fossil fuels and industrial processes, is a primary pollutant. When it combines with other compounds in the atmosphere, it undergoes a series of chemical reactions and forms sulphuric acid. Therefore, sulphuric acid is a secondary pollutant.

Carbon Dioxide:
Carbon dioxide (CO2) is primarily emitted from burning fossil fuels and deforestation. It is considered a primary pollutant because it is directly released into the atmosphere. Carbon dioxide is a greenhouse gas and contributes to global warming, but it is not considered a secondary pollutant.

Ozone:
Ozone (O3) is a secondary pollutant that is formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight. These primary pollutants are emitted from vehicles, power plants, and industrial processes. Ozone is a major component of smog and can cause respiratory problems and other health issues.

Carbonic Acid:
Carbonic acid (H2CO3) is formed when carbon dioxide dissolves in water. While carbonic acid can contribute to the acidity of rainwater, it is not considered a significant secondary pollutant compared to other pollutants like sulphuric acid and ozone.

Conclusion:
From the given options, carbon dioxide (option B) is not a secondary pollutant. It is a primary pollutant released directly into the atmosphere. Sulphuric acid, ozone, and carbonic acid are all secondary pollutants formed through chemical reactions involving primary pollutants.