Which of the following gases indicates the health of an engine through its emissions?

Considering the health of an engine through its emissions, it's important to recognize that each of these gases provides valuable insights into the combustion process and overall engine performance. Carbon dioxide (CO2) is a byproduct of complete combustion, and higher levels can suggest efficiency in fuel usage. Oxygen (O2) levels indicate how well the engine is breathing; for example, higher O2 levels might suggest incomplete combustion or that the air-fuel mixture is too lean. Carbon monoxide (CO) is a critical indicator of incomplete combustion, where fuel is not fully burned, which can signal issues in the engine’s efficiency and potentially harmful emissions.

When assessing an engine's health, monitoring all these gases—CO2, O2, and CO—gives a more comprehensive view of performance and emissions, allowing for more effective adjustments and repairs to enhance engine operation. Thus, recognizing the importance of all three gases collectively makes it clear why this option is the most suitable choice.