Given data

• Mass percent of $\mathrm{H_2SO_4}$ = $31.4\%$
• Density of solution = $1.25\,\mathrm{g/mL}$
• Molar mass of $\mathrm{H_2SO_4}$ = $98\,\mathrm{g/mol}$

Take 1 litre of solution Since molarity is moles per litre, consider $1000\,\mathrm{mL}$ of solution.

Mass of $1$ L solution: $1000 \times 1.25 = 1250\,\mathrm{g}$

Find mass of $\mathrm{H_2SO_4}$ in this solution Since the solution is $31.4\%$ by mass, $\text{Mass of } \mathrm{H_2SO_4} = \frac{31.4}{100} \times 1250$ $= 392.5\,\mathrm{g}$

Convert mass into moles $\text{Moles of } \mathrm{H_2SO_4} = \frac{392.5}{98}$ $= 4.0051\approx 4.01$

Calculate molarity Since this is in $1$ litre of solution, $\text{Molarity} = 4.01\,\mathrm{M}$

Nearest integer $\boxed{4}$