The correct option here is :

Option A : catalytic agent

In the Kjeldahl method for the estimation of nitrogen, $\mathrm{CuSO}_4$ or copper sulfate acts as a catalytic agent. The method consists of three main steps: digestion, neutralization, and titration. During the digestion phase, the organic substance containing nitrogen is heated with concentrated sulfuric acid ($\mathrm{H_2SO_4}$), which digests or breaks down the organic matrix, releasing the nitrogen in the form of ammonium sulfate ($\mathrm{(NH_4)_2SO_4}$).

Organic compound $+\mathrm{H}_2 \mathrm{SO}_4 \xrightarrow{\mathrm{CuSO}_4}\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4$

Catalysts such as $\mathrm{CuSO}_4$, along with other salts like potassium sulfate ($\mathrm{K_2SO_4}$), are added to the reaction mixture to raise the boiling point of the sulfuric acid and to accelerate the oxidation of the organic material. This increases the rate of the digestion process, ensuring that the nitrogen within the sample is fully converted to ammonium ions ($\mathrm{NH_4}^+$). The catalyst itself is not consumed in the reaction and does not become a part of the product; its role is to increase the reaction rate. After digestion, the mixture is neutralized with a base, and then the liberated ammonia is distilled off and quantitatively measured, typically by titration with a standard acid solution.

Therefore, $\mathrm{CuSO}_4$ serves as a catalyst in this chemical analysis technique for quantitatively determining the nitrogen content of organic substances.