The structure of the major product formed in the following reaction is:

Step 1: Identify the reaction

Aryl/alkyl halide + AgCN → cyanide substitution

AgCN can act as a nucleophile where $\ce{CN-}$ replaces the halogen.

Step 2: Determine which halogen is replaced

The substrate has:

• Br at one position (on benzene ring)
• Cl at another position (on benzene ring or side chain)

For nucleophilic substitution:

• Reactivity order: I > Br > Cl > F
• Better leaving group = faster substitution

But wait, for aryl halides (halogen directly on benzene ring):

• Nucleophilic aromatic substitution ($S_NAr$) is very difficult
• Requires strong electron-withdrawing groups (like $\ce{NO2}$) to activate the ring
• Without activation, aryl halides don't undergo simple $S_N2$ or $S_N1$

If the structure has:

• Br on the benzene ring (aryl bromide) - unreactive
• Cl on a side chain (alkyl chloride) - reactive

Then $\ce{CN-}$ will replace the alkyl Cl, not the aryl Br.

Step 3: Determine the product

Major product: CN replaces Cl (more reactive position), Br remains on the ring.

Answer: (b)

Key Points:

• Aryl halides (X on benzene ring) are unreactive toward simple nucleophilic substitution
• Alkyl halides (X on side chain) undergo $S_N2$/$S_N1$ readily
• Leaving group ability: I > Br > Cl > F (for alkyl halides)
• $S_NAr$ requires activation by electron-withdrawing groups