What maintenance item can directly affect a truck’s emission control system?

Regular replacement of the air filter is crucial for maintaining optimal performance in a truck's engine, which in turn directly affects the emission control system. The air filter's primary function is to prevent dirt, dust, and other debris from entering the engine. If the air filter becomes clogged or dirty, it restricts the airflow into the engine, which can lead to a lean air-fuel mixture. This condition may cause the engine to operate less efficiently and potentially increase emissions of harmful pollutants.

When the engine is not receiving enough clean air, it may not burn fuel completely, leading to incomplete combustion and the production of greater amounts of hydrocarbons and carbon monoxide. Properly maintaining the air filter ensures that the engine can operate within its designed parameters, thus supporting the emission control system's functionality, which is designed to reduce harmful emissions.

The other options relate to maintenance practices that do not have a direct impact on the emission control system. For instance, washing the truck body primarily affects aesthetics and does not influence engine performance or emissions. Tire rotation contributes to tire longevity and safety but does not directly influence emission levels. Checking the brake fluid level is important for brake operation and safety but does not have any relation to emission control. Thus, regular replacement of the air filter stands