Which of the following exhibits non-stoichiometric metal deficiency?

Metal deficiency defect is a type of non-stoichiometric defect where the number of metal cations is less than the number of anions in the crystal lattice.

To maintain electrical neutrality, the missing positive charge is compensated by an adjacent metal ion acquiring a higher oxidation state.

This defect can only be shown by metals that exhibit variable valency or multiple oxidation states.

Since transition metals are well-known for exhibiting variable oxidation states (e.g., $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$ in $\text{FeO}$, or $\text{Ni}^{2+}$ and $\text{Ni}^{3+}$ in $\text{NiO}$), they commonly show non-stoichiometric metal deficiency defects.

Alkali and alkaline earth metals have fixed oxidation states ($+1$ and $+2$, respectively) and therefore do not exhibit this defect.

Answer: Transition metals only