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

The correct choice is the electrode, as it is a critical component of the spark plug. The electrode is responsible for creating a spark that ignites the air-fuel mixture in the engine's combustion chamber. This spark occurs when high voltage from the ignition system passes through the electrode, producing a spark gap that ignites the fuel.

The other components listed, such as the piston, valve, and fuel injector, serve different functions within the engine. The piston is involved in the engine's compression and power strokes, valves control the intake and exhaust flow in the engine, and fuel injectors are responsible for delivering fuel into the combustion chamber. Understanding the specific roles of these components highlights the specialized function of the spark plug and its essential role in engine operation.