In a spark-ignition engine the spark plug lights the compressed air-fuel mixture, and a flame front should then travel smoothly outward, burning the charge in a controlled way.

As this flame advances across the chamber it compresses and heats the portion of mixture still ahead of it, known as the end gas.

If the end gas becomes hot and pressurised enough, it can ignite spontaneously on its own before the normal flame reaches it, releasing its energy almost instantly.

This near-instant burn produces a sharp pressure spike and a shock wave that strikes the cylinder walls, heard as the metallic rap of knocking.

Because a higher compression ratio raises end-gas temperature and pressure and so makes auto-ignition more likely, the fear of knock forces designers to cap the compression ratio of SI engines.

The other options are not the primary cause: poor vaporisation (option 1) causes rough running rather than knock, excess oxygen (option 3) leans the mixture but does not trigger end-gas detonation, and hot lubricating oil (option 4) is a symptom of engine heat, not the source of knock.

Hence, the answer is Auto-ignition of unburned mixture ahead of flame front.