If M is the molar mass of P₄, then its equivalent mass in the reaction is 
P₄ + OH^- ----> PH₃ + H₂PO₂ ^-
a) M/12
b) M/4
c) M/3
​d) M

Dear Student
We know equivalent mass = (molecular mass)/oxidation state changed
In the above reaction first oxidation number was = 0
After the reaction oxidation number of P in PH3 become -3 as oxidation number of hydrogen is +1. And oxidation number in  H₂PO₂ ^-  is +3 as oxidation number of oxygen = -2.
So equivalent mass = molecular mass/3
                           =M/3
So answer C is the correct answer.



Regards