if the molecular weight of H3PO3 is M, its equivalent weight will be - a] M B] M/3 C] M/2 D] 2M

option c is correct
Equivalent mass of an acid = molecular weight of the acid/basicity of the acid

Now the real question here is what is the basicity of H3PO3.

For an oxy acid its the number of hydrogen attached to oxygen in the compound.

For H3PO4 it is 3.

For H3PO3 it is 2.

Therefore equivalent weight = M/2