Which of the following can reduce engine performance?

A clogged air filter can significantly reduce engine performance because it restricts the airflow into the engine. The engine requires a specific mixture of air and fuel to operate efficiently, and when the air filter is clogged, it prevents sufficient air from reaching the combustion chambers. This can lead to a richer fuel mixture, making the engine run less efficiently, reducing power, and increasing fuel consumption.

Properly tuned components, such as the battery and high-quality engine oil, contribute to optimal engine performance by ensuring that electrical systems function well and that the engine is lubricated effectively. Clean fuel injectors ensure that the right amount of fuel is delivered to the engine, which is essential for maintaining performance. Hence, while all these factors are crucial for engine health, a clogged air filter directly impairs the engine's ability to breathe properly, leading to a noticeable drop in performance.