During the preparation of Mohr’s salt solution (Ferrous ammonium sulphate), which of the following acid is added to prevent hydrolysis of ${\text{Fe}}^{2+}$?

Mohr's salt is ${\text{FeSO}}_4\cdot ({\text{NH}}_4{)}_2{\text{SO}}_4\cdot 6{\text{H}}_2\text{O}$.In aqueous solution, ${\text{Fe}}^{2+}$ ions undergo hydrolysis to form ${\text{Fe(OH)}}_2$, which makes the solution turbid. To prevent this hydrolysis and keep the iron in its $+2$ oxidation state, dilute sulphuric acid (${\text{H}}_2{\text{SO}}_4$) is added.