Which of the following gases tends to decrease when proper combustion occurs?

When proper combustion occurs, the primary goal is to maximize the conversion of fuel into energy while minimizing the production of harmful byproducts. One of the main byproducts of incomplete combustion is carbon monoxide (CO), a toxic gas that is typically formed when there is not enough oxygen available for complete combustion to occur.

In ideal conditions, adequate oxygen is supplied, and fuel is combusted completely, resulting in carbon dioxide (CO₂) being produced instead of CO. This transition from CO to CO₂ indicates that the combustion process is occurring efficiently, utilizing the fuel fully and reducing the volume of harmful emissions. As a result, a decrease in CO correlates with proper combustion.

While other gases listed, such as hydrocarbons (HC) and oxygen (O₂), may also be affected in this process, the specific focus on CO highlights its direct relationship with combustion efficiency. Proper combustion aims to minimize CO emissions, making it the gas that typically decreases when combustion is executed correctly.