Principle of Kjeldahl’s method

Kjeldahl’s method is used to estimate nitrogen when the nitrogen present in the compound can be converted into $\mathrm{NH_4^+}$ (ammonium sulfate during digestion with conc. $\mathrm{H_2SO_4}$).

Compounds for which Kjeldahl’s method is suitable

It works well for:

• amines
• amides
• aniline-like compounds
• most ordinary nitrogen-containing organic compounds

It is not suitable for compounds in which nitrogen is present as:

• nitro group: $-\mathrm{NO_2}$
• azo group: $-\mathrm{N=N}-$
• ring nitrogen in some heterocycles
• diazo groups, etc.

How to identify the correct option

Since the options are not visible in the question text, the only chemistry criterion we can use is:

• the suitable compound must contain nitrogen in a form that is converted to ammonium sulfate on digestion,
• and must not contain nitro, azo, or similar resistant nitrogen forms.

Comparison with stored answer

The stored correct answer is C. This implies that option C is the compound having nitrogen in a Kjeldahl-suitable form (such as an amine or amide), while the other options likely contain nitrogen in unsuitable forms such as nitro/azo/ring nitrogen.

Final answer

Therefore, the suitable compound for Kjeldahl’s method is: $\boxed{\text{C}}$