Reaction Mechanism:
The reaction of propene with HBr in the presence of peroxide proceeds via a radical mechanism known as free radical addition. This mechanism involves the following steps:

1. Initiation: The peroxide (ROOR) is homolytically cleaved by heat or light to generate two reactive radicals (RO•). This step provides the necessary radicals for the subsequent steps.

2. Propagation:
- The first propagation step involves the reaction between propene (CH2=CHCH3) and one of the peroxide radicals (RO•). The radical attacks the double bond of propene, forming a new carbon-centered radical intermediate.
- The second propagation step involves the reaction of the newly formed propyl radical with HBr. The radical abstracts a hydrogen atom from HBr, resulting in the formation of 1-bromopropane and a new bromine-centered radical.

3. Termination: The reaction terminates when two radicals combine to form a stable compound or when all the radicals are consumed.

Explanation:
In the given reaction, propene (CH2=CHCH3) reacts with HBr in the presence of peroxide. The reaction proceeds via a radical mechanism, where a bromine atom is added to the double bond of propene.

- The reaction starts with the initiation step, where the peroxide (ROOR) is homolytically cleaved to generate two reactive radicals (RO•).
- In the propagation step, one of the peroxide radicals attacks the double bond of propene, forming a new carbon-centered radical intermediate. This intermediate reacts with HBr, abstracting a hydrogen atom and forming a new bromine-centered radical. As a result, 1-bromopropane is formed.
- The other propagation steps involving the second peroxide radical will result in the formation of other products, such as 2-bromopropane or 3-bromopropane. However, the reaction conditions (presence of peroxide) favor the formation of 1-bromopropane.
- The termination step can occur when the radicals combine to form stable compounds or when all the radicals are consumed.

Therefore, the correct product formed in the reaction of propene with HBr in the presence of peroxide is 1-bromopropane (option B).