Principle of Kjeldahl’s method

Kjeldahl’s method is used to estimate nitrogen when the nitrogen present in the compound can be converted into ammonium sulfate on digestion with concentrated $\mathrm{H_2SO_4}$.

It does not work for nitrogen present in certain forms such as:

• nitro group: $-\mathrm{NO_2}$
• azo group: $-\mathrm{N=N}-$
• ring nitrogen in some heterocyclic compounds

Check each option

A. $C_6H_5NH_2$ (aniline)
Nitrogen is present as an amino group $(-\mathrm{NH_2})$. This is estimated by Kjeldahl’s method.
So, A can be used.

B. $\mathrm{NH_2CONH_2}$ (urea)
Nitrogen is present in amide-type form, which is converted to ammonium sulfate during digestion.
So, B can be used.

C. $C_6H_5NO_2$ (nitrobenzene)
Nitrogen is present in the nitro group $(-\mathrm{NO_2})$. Kjeldahl’s method cannot estimate nitrogen in nitro compounds.
So, C cannot be used.

D. $CH_3CH_2C\equiv N$ (propionitrile)
Nitrogen is present as a cyano group. In the usual JEE-level exceptions for Kjeldahl’s method, nitro, azo, and ring nitrogen are excluded; nitriles are not the standard exception here.
So, D can be used.

Conclusion

The compound for which Kjeldahl’s method cannot be used is: $\boxed{C_6H_5NO_2}$

Comparison with stored correct answer

Stored correct answer: C
Derived answer: C
Therefore, they agree.