In the reaction P₄ + 3OH^– + 3H₂O →3H₂PO₂^– + PH₃; phosphorus is undergoing

$\stackrel{0}{{\mathrm{P}}_4}+3{\mathrm{OH}}^{-}+3{\mathrm{H}}_2\mathrm{O}\to \stackrel{-3}{\mathrm{P}}{\mathrm{H}}_3+3{\mathrm{H}}_2\stackrel{+1}{\mathrm{P}}{\mathrm{O}}_2^{-}$

Phosphorus undergoes reduction from P₄ to PH₃ and undergoes oxidation when it forms from H₂PO₂^– to P₄. Hence, option-C is correct.