Among the following the strongest nucleophile is:

Nucleophiles are species that can donate a pair of electrons. Nucleophilic power depends on the tendency of the species to donate electrons.${\text{NCCH}}_2^{-}$ has a negative charge and an electron-pushing group ($+I$ group) which increases its ability to donate electrons compared to neutral species or those with electron-withdrawing groups.Species with lower electronegativity for the donor atom are generally stronger nucleophiles ($\text{C}>\text{N}>\text{O}$).Therefore, ${\text{NCCH}}_2^{-}$ is the strongest nucleophile among the given options.