Which of the following is a part of a spark plug?

The terminal nut is indeed a part of a spark plug and plays a crucial role in its function. The terminal nut serves as the point where the ignition cable connects to the spark plug, allowing electrical current to flow from the ignition system to the plug itself. When current flows through the spark plug, it creates a spark that ignites the air-fuel mixture in the engine's combustion chamber, enabling the engine to run.

In contrast, while the other options may relate to engine components, they do not constitute parts of a spark plug. The cap could refer to various caps found in engines but isn’t specific to spark plugs. The spark coil is a separate component responsible for transforming voltage, creating the necessary spark for ignition, but it is not part of the spark plug itself. The cylinder is an essential part of the engine where combustion takes place, but it does not relate to the structure of a spark plug. Understanding the functions and components of the spark plug is vital for diagnosing ignition issues and performing maintenance on small engines.