The organic compound used as antiknock agent in petroleum is
Correct option is A. $$(C_2H_5)_4Pb$$
This abnormal combustion produces a specific type of sound, which we call $$knocking. $$
Knocking has adverse effects. It results in early wear-and-tear of the piston. Carbon is deposited in the inner part of the cylinders [Internal Combustion Engines have cylinders, inside which pistons move]. Also, Efficiency is reduced.
So we want to reduce knocking. We add something which can reduce it. Such substances are called $$Anti-knocking Agents.$$
A popular anti-knocking agent was $$\text{Tetra Ethyl Lead (TEL)}(C_2H_5)_4Pb. $$