A mixture containing two enantiomers in equal proportions but with zero optical activity because the opposite optical rotations of the two enantiomers cancel out each other, is called a racemic mixture. For a racemic mixture to occur after nucleophilic substitution, the optically active reactant undergoes S_N1 reaction. Options (a) is a chiral carbon atom and it will undergo S_N1 mechanism to give a racemic mixture. Option (b) is not an asymmetric carbon, and option (c) contains a secondary carbon asymmetric atom, which has less reactivity towards an S_N1 substitution. The correct option is (i).