In a spark ignition engine, the spark plug is meant to be the only source of ignition, lighting a flame front that burns smoothly across the fuel-air charge. Raising the compression ratio squeezes the unburned mixture to a higher temperature and pressure before the spark even fires. If the compression ratio is pushed too high, the last pockets of unburned mixture ahead of the flame front reach their own self-ignition temperature on their own, well before the flame front arrives, and they explode spontaneously all at once. This sudden, uncontrolled explosion sends sharp pressure waves hammering against the cylinder walls and piston, producing the characteristic metallic pinging sound called knocking. Because knocking can damage the piston, rings and cylinder head if it happens repeatedly, engine designers keep the compression ratio of petrol engines within a safe upper limit to avoid it. So the limiting factor is knocking, option (C).