The order of decreasing nucleophilicity is

Due to presence of –ve charge on the oxygen atom. ${\mathrm{CH}}_3{\mathrm{O}}^{-}$ , ${\mathrm{HO}}^{-}$ and ${\mathrm{CH}}_3{\mathrm{CCO}}^{-}$ ions are more nucleophillic than ${\mathrm{H}}_2\mathrm{O}$ . Further, due to +I-effect of the ${\mathrm{CH}}_3$ group ${\mathrm{CH}}_3{\mathrm{O}}^{-}$ is more nucleophilic than ${\mathrm{OH}}^{-}$ . Further since in ${\mathrm{CH}}_3{\mathrm{CCO}}^{-}$ , -ve charge on oxygen is stabilized by resonance, therefore, its nucleophilicity is lower than those of  ${\mathrm{CH}}_3{\mathrm{O}}^{-}$ and ${\mathrm{OH}}^{-}$  . Thus , the overall nucleophilicity of all the species decreases in the order:  ${\mathrm{CH}}_3{\mathrm{O}}^{-}>{\mathrm{OH}}^{-}>{\mathrm{CH}}_3{\mathrm{COO}}^{-}>{\mathrm{H}}_2\mathrm{O}$i.e., option (d) is correct