Correct option is (b) NH3
In Kjeldhal's method, the compound containing nitrogen is heated with concentrated sulphuric acid and copper sulphate. Nitrogen in the compound gets converted into ammonium sulphate.
Organic compound + H2SO4 ⟶ (NH4)2SO4
The resulting acid mixture is then heated with an excess of sodium hydroxide to produce sodium sulphate and ammonia.
(NH4)2SO4 + 2NaOH → Na2SO4 + 2NH3 + 2H2O
The liberated ammonia gas is absorbed in a standard solution of known volume of excess sulphuric acid.
The amount of ammonia produced is determined by estimating the amount of sulphuric acid consumed in the reaction.
The amount of unreacted sulphuric acid left after the absorption of ammonia is estimated by titrating it with a standard alkali solution.
Thus, nitrogen present in the organic compound is estimated as NH3 in Kjeldahl's method.
