The given reaction is an S_N2 reaction, a nucleophilic substitution. Alkyl halides undergoing S_N2 reaction undergo inversion of configuration, as the incoming nucleophile causes the groups surrounding the carbon atom move in a direction opposite to that of the nucleophile. Hence, (b) and (d) will have opposite configuration. This reaction is also S_N2 because it has only one step where addition of OH^- and removal of Cl^- is simultaneous. The correct options are (i) and (ii).