d
Nucleophilic substitution bimolecular \(S_{N^{2}}\) prefers less sterically hindered site to attack. Lesser the steric hindrance faster the \(S_{N^{2}}\) reaction. So ease of reaction is \(1^{o}>2^{o}>3^{o}\)

\(S_{N^{2}}\) involves inversion of configuration stereo, chemically. since, \(1^{o}\) alkyl halides are prone to \(S_{N^{2}}\) reactions, therefore \(\mathrm{CH}_{3} \mathrm{Cl}\) undergoes complete strerochemical inversion.