Given, Density= $1.25 \mathrm{g}/\mathrm{mL}$,

Molar mass of ${\mathrm{H}}_2{\mathrm{SO}}_4=98 \mathrm{g} {\mathrm{mol}}^{-1}$

Molarity is defined as no. of moles of solute present in one litre of solution. Thus,

Molarity= $\frac{\mathrm{No}. \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{solute}}{\mathrm{Volume} \mathrm{of} \mathrm{solution} \mathrm{in} \mathrm{L}}$

No. of moles= mass/ molar mass

$\mathrm{M}=\frac{{\mathrm{n}}_{\text{solute }}}{\mathrm{V}}\times 1000$

$=\frac{\left(\frac{31.4}{98}\right)}{\left(\frac{100}{1.25}\right)}\times 1000$

$=4.005\approx 4$