In Kjeldahl's method for estimation of nitrogen, CuSO _ 4 acts as :

Kjeldahl's method is used for the estimation of nitrogen. The organic compound is heated with conc. ${H}_{2}{SO}_{4}$ in presence of ${K}_{2}{SO}_{4}$ and a little ${CuSO}_{4}$ to convert all the nitrogen into ${\left({NH}_{4}\right)}_{2}{SO}_{4} . {K}_{2}{SO}_{4}$ raises the boiling point of ${H}_{2}{SO}_{4}$ while ${CuSO}_{4}$ catalyses the reaction.

Hence, B is the answer.