c
Kjeldahl's method is not applicable for cpmpounds containing nitrogen in nitro and azo group and nitrogen present in the ring. Because nitrogen of these compounds does not change to ammonium sulphate under these conditions.

Hence only Alanine can be used to determine percentage of nitrogen. 

\(\underset{Alanine}{\mathop{\begin{matrix}
   {{H}_{2}}N-CH-COOH  \\
   |\,\,\,\,\,\,\,\,\,  \\
   \,\,\,\,\,\,\,\,\,CH{{(C{{H}_{3}})}_{2}}  \\
\end{matrix}}}\,\)