Kjeldahl's method digests an organic compound with concentrated H${}_2$SO${}_4$ to convert organically bound nitrogen into ammonium sulphate, which is then liberated as NH${}_3$ and titrated.

The method is only applicable when nitrogen is present in a form that can be converted to ammonium sulphate during digestion. It fails for nitrogen present in nitro groups, azo groups, or in aromatic rings (such as pyridine), because such nitrogen is not converted to NH${}_3$ under these conditions.

Among the options, only aniline has nitrogen in an amino group that is readily converted to (NH${}_4$)${}_2$SO${}_4$, making Kjeldahl's method suitable.