3.92 g. of Mohr salt [mol. wt = 392] is present in 100 ml of an aqueous solution. The sulphate ion concentration of the resulting solution is

Mohr salt mass, given as 3.92g
Mohr salt's molecular weight is 392 g/mol.
100 mL is the solution's volume.

$\mathrm{M}=\frac{\mathrm{Wt}.}{\mathrm{Mol}.\mathrm{wt}.}\times \frac{1000}{\mathrm{V}\left(\mathrm{ml}\right)}=\frac{3.92}{392}\times \frac{1000}{100}=0.1\mathrm{M}$

The chemical formula for ionising mohr salt is as follows:

${\left({\mathrm{NH}}_4\right)}_2\mathrm{Fe}{\left({\mathrm{SO}}_4\right)}_2.6{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{NH}}_4^{+}+{\mathrm{Fe}}^{+2}+2{\mathrm{SO}}_4^{-2}+6{\mathrm{H}}_2\mathrm{O}$

We may state that the sulphate ion concentration = (2×0.1)=0.2M

Hence the correct option is (C) 0.2M