Option 1: Breathing involves the exchange of gases, including the release of carbon dioxide (CO2) from the body into the air. This process increases the amount of CO2 in the air.

Option 2: Photosynthesis is the process by which plants convert CO2 into oxygen (O2) using sunlight. This process actually decreases the amount of CO2 in the air, as it removes CO2 and releases O2.

Option 3: Burning of petrol releases CO2 into the air as a byproduct of combustion. This process increases the amount of CO2 in the air.

Option 4: Aerobic decay of vegetation occurs when organic material decomposes in the presence of oxygen. This process releases CO2 into the air, increasing the amount of CO2.

Therefore, the correct answer is option 2. Alert - correct answer should be option 1.