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

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