Q.Which of the following is likely to undergo racemization during alkaline hydrolysis ?

Two mechanisms carry out nucleophilic substitution, distinguished by kinetics, stereochemistry and which substrates favour each. SN2 (bimolecular): Rate = k2[RX][Nu-], second order, ONE step. The nucleophile attacks from directly opposite the leaving halogen (backside attack) through a single transition state, inverting the carbon's configuration exactly as an umbrella flips in a strong wind (Walden inversion) -- so an optically active substrate with the leaving group ON the stereocentre gives a single inverted, still optically active product. SN2 favours LESS hindered carbons: methyl > primary > secondary >> tertiary (tertiary essentially never goes SN2, since three alkyl groups block the required backside approach). SN1 (unimolecular): Rate = k[RX] only, independent of nucleophile concentration, first order, TWO steps -- (1) slow, rate-determining heterolysis of C-X to a planar carbocation (sp2, empty p-orbital with two equal lobes), then (2) fast attack by the nucleophile from EITHER face. Because both faces are attacked with roughly equal probability, a chiral SN1 substrate (halogen on the stereocentre) gives a racemic (optically inactive) mixture. SN1 favours MORE substituted carbons: tertiary > secondary > primary > CH3X, since more alkyl groups (inductive donation plus hyperconjugation) stabilise the intermediate carbocation. A frequent trap: a molecule can be chiral overall yet have its halogen on a DIFFERENT carbon from the one bearing three-plus different groups -- if the halogen sits on a primary carbon elsewhere in the molecule, that primary centre still reacts by SN2 (inversion at THAT carbon), while the genuine, untouched stereocentre elsewhere keeps its original configuration and the product is optically ACTIVE, not racemic. Racemisation specifically requires the leaving group to sit ON the carbon that is (or becomes, via the planar cation) the stereocentre.