The correct answer is C. tetraethyl lead.
Tetraethyl lead is an organometallic compound with the formula (C2H5)4Pb. It is a colorless, flammable liquid that is used as an antiknock agent in gasoline. Tetraethyl lead is added to gasoline to prevent engine knocking, which is a phenomenon that occurs when the air-fuel mixture in an engine ignites prematurely. This can cause the engine to lose power and can also damage the engine.
Ethyl alcohol is a colorless, flammable liquid that is used as a fuel, solvent, and antiseptic. It is not an antiknock agent and does not prevent engine knocking.
Butane is a colorless, flammable gas that is used as a fuel, refrigerant, and aerosol propellant. It is not an antiknock agent and does not prevent engine knocking.
White petrol is a type of gasoline that is clear and colorless. It is not an antiknock agent and does not prevent engine knocking.