Concept:
In the presence of peroxide, addition of HBr to an alkene proceeds through a free radical mechanism and follows anti-Markovnikov orientation. This is known as the Peroxide Effect or Kharasch Effect.
Step 1: Identify the alkene.
The substrate is allyl benzene.
\[
\mathrm{C_6H_5CH_2CH=CH_2}
\]
Step 2: Apply anti-Markovnikov addition.
Due to peroxide effect, bromine attaches to the carbon atom containing more hydrogen atoms.
Therefore bromine adds to the terminal carbon atom.
Step 3: Write the product.
\[
\mathrm{C_6H_5CH_2CH_2CH_2Br}
\]
Hence option (B) is correct.
