Kjeldahl's method is used for the estimation of nitrogen. The organic compound is heated with conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ in presence of ${\mathrm{K}}_2{\mathrm{SO}}_4$ and a little ${\mathrm{CuSO}}_4$ to convert all the nitrogen into ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4.{\mathrm{K}}_2{\mathrm{SO}}_4$ raises the boiling point of ${\mathrm{H}}_2{\mathrm{SO}}_4$ while ${\mathrm{CuSO}}_4$ catalyses the reaction.

Hence, B is the answer.