Q.A benzene ring bearing a -CH2-CH=CH2 (allyl) side chain $\xrightarrow{HCl}$ (A) is

Markovnikov's Rule

Markovnikov's rule predicts the regiochemistry of adding an unsymmetrical reagent (H-X, H-OH via H3O+, H-OSO3H, etc.) across an unsymmetrical alkene's double bond: the hydrogen (or, more generally, the less electronegative/electrophilic part of the reagent) adds to the alkene carbon that already carries MORE hydrogens; the halogen/nucleophile adds to the carbon carrying FEWER hydrogens. Equivalently: the most electronegative part of the reagent ends up on the more-substituted double-bond carbon.

The mechanistic reason is carbocation stability. Protonation (or analogous electrophilic attack) can occur at either alkene carbon, generating either a less-substituted or a more-substituted carbocation; since alkyl groups stabilise a positive charge by hyperconjugation and induction, the MORE substituted carbocation (tertiary > secondary > primary) forms preferentially and lives long enough to be captured by the nucleophile -- so the nucleophile ends up bonded to the more-substituted carbon, exactly where Markovnikov's rule places it. Worked for propene + HBr: protonating the terminal CH2 gives a secondary carbocation at C2 (favoured over protonating C2 to give a primary carbocation at the terminal carbon); Br- then attacks this secondary cation, giving 2-bromopropane …