Which of the following factors can lead to increased emissions?

C is the correct choice because old spark plugs and a clogged air filter can significantly impede the engine's performance, leading to incomplete combustion and an increase in harmful emissions. Spark plugs are responsible for igniting the air-fuel mixture in the engine cylinder. If they wear out or fail, they can cause misfires, leading to inefficient fuel burning and higher hydrocarbon emissions. Similarly, a clogged air filter restricts airflow to the engine, which disrupts the ideal air-fuel mixture necessary for optimal combustion. When the engine cannot access enough clean air, it can result in richer fuel mixtures, incomplete combustion, and higher levels of emissions such as carbon monoxide and particulate matter.

The other factors listed do not lead to increased emissions. Regular engine oil changes help maintain engine health and efficiency, while proper tire inflation ensures optimal fuel efficiency and reduces emissions associated with excess fuel consumption. New fuel injectors typically improve engine performance by delivering the correct amount of fuel more efficiently, thus reducing harmful emissions.