Reaction:
The reaction between hydrochloric acid gas and propene in the presence of benzoyl peroxide is an example of an electrophilic addition reaction.

Electrophilic Addition Reaction:
An electrophilic addition reaction involves the addition of an electrophile to a molecule, resulting in the formation of a new compound. In this case, the electrophile is the positively charged hydrogen atom in hydrochloric acid.

Benzoyl Peroxide:
Benzoyl peroxide is a radical initiator that is commonly used in organic chemistry reactions. It provides free radicals that initiate the reaction by abstracting a hydrogen atom from a reactant.

Propene:
Propene is an unsaturated hydrocarbon with a double bond between two of its carbon atoms. It is an alkene and can undergo addition reactions.

Step 1: Initiation
Benzoyl peroxide undergoes homolytic cleavage to form benzoyl radicals. Each benzoyl radical then abstracts a hydrogen atom from hydrochloric acid gas, resulting in the formation of a chlorine radical and a benzoyl chloride radical.

Step 2: Propagation
The chlorine radical reacts with propene by abstracting a hydrogen atom from one of its carbon atoms. This generates a propene radical, which is a highly reactive species.

Step 3: Termination
The propene radical can react with the chlorine radical to form 2-chloropropane. This is the desired product in this reaction.

Explanation of Correct Answer:
The correct answer is option 'A' - 2-chloropropane. This is because the reaction between hydrochloric acid gas and propene in the presence of benzoyl peroxide leads to the formation of 2-chloropropane as the main product.

The reaction proceeds through a radical mechanism, where benzoyl peroxide acts as a radical initiator. The benzoyl peroxide undergoes homolytic cleavage to form benzoyl radicals, which then abstract a hydrogen atom from hydrochloric acid gas. This results in the formation of a chlorine radical and a benzoyl chloride radical.

The chlorine radical then reacts with propene, abstracting a hydrogen atom from one of its carbon atoms. This generates a propene radical, which is highly reactive. The propene radical can further react with the chlorine radical, leading to the formation of 2-chloropropane.

Therefore, the correct product formed in this reaction is 2-chloropropane, which corresponds to option 'A'.