When inspecting a spark plug, what does a dry, fluffy black deposit on the electrodes and insulator indicate about the engine?

1. The engine is running with a rich fuel mixture (too much fuel or restricted air intake).
2. The engine is running lean (too little fuel or vacuum leaks).
3. The cylinder is leaking oil past worn valve guide seals.
4. The spark plug gap is adjusted too narrow, causing weak ignition.