Plz answer in detaaaillll

Dear student

• Since the S_N2 proceeds through a backside attack, if a stereocenter is present the S_N2 reaction will give inversion of stereochemistry.
• By contrast, if the S_N1 leads to the formation of a stereocenter, there will be a mixture of retention and inversion since the nucleophile can attack from either face of the flat carbocation

So we will find from the above options which is most favorable for S_N1 
 S_N1 formation takes when the carbocation formed is very stable so in option (3) there is formation of benzylic carbocation which is very stable hence that will be the correct option.

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