Which of the following can cause a misfire when the engine is under load?

A cracked spark plug can indeed cause a misfire under load because it creates an interruption in the spark path, leading to incomplete combustion. When the engine is under load, it requires a more efficient and robust spark to ignite the air-fuel mixture. A crack in the spark plug can create a gap that does not properly ignite the mixture, causing a misfire, particularly when the engine is stressed.

While a foul spark plug could also lead to misfiring due to fouling-related difficulties in creating a spark, it might not be as immediate or severe under load as a physical crack. Foul spark plugs often lead to misfires at idle or lower RPMs when engine loads are lighter, whereas a cracked spark plug is more likely to fail when the demands are higher. Understanding the nature of engine load is crucial in diagnosing misfire issues, making it essential to evaluate both conditions while considering the correct choice.