Option 1: Adding ethylene dibromide to petrol does not directly increase the octane number of the fuel. The octane number is a measure of the fuel`s ability to resist knocking or pinging in a combustion engine. It is typically increased by adding substances such as tetra-ethyl lead (TEL) or other octane boosters.

Option 2: This is the correct answer. Adding ethylene dibromide to petrol can help with the elimination of lead oxide. Lead oxide is a byproduct of using TEL as an octane booster in petrol. Ethylene dibromide reacts with lead oxide to form a stable lead bromide compound, which can be easily eliminated from the fuel system, reducing the harmful effects of lead pollution.

Option 3: Ethylene dibromide does not have a significant role in removing sulfur compounds from petrol. Sulfur compounds are typically removed through other refining processes.

Option 4: Ethylene dibromide cannot serve as a direct substitute for tetra-ethyl lead in boosting the octane number of fuel. TEL is a highly effective octane booster, and while there are alternative compounds that can increase the octane number, ethylene dibromide is not one of them.

In conclusion, adding eth