Kjeldahl's method works by digesting the organic compound with concentrated sulphuric acid so that nitrogen is quantitatively converted into ammonium sulphate, from which ammonia is later liberated and estimated. The method is reliable only when the nitrogen can be smoothly converted into ammonium ion during digestion. Nitrogen present in nitro groups, azo groups, or in aromatic rings such as pyridine is not fully converted into ammonium sulphate under these conditions, so the method underestimates nitrogen and is therefore not applicable to such compounds. Nitrogen in simple amine groups is readily converted to ammonium sulphate and is well suited to the method, so that option is wrong. Amide nitrogen is also hydrolysed and digested successfully, giving accurate results, so it does not represent a failure case. Ammonium salts already contain nitrogen as ammonium ion and pose no problem for the method. The exclusion of nitro, azo, and ring nitrogen is the standard NCERT limitation of Kjeldahl's method, and for such compounds Duma's method is preferred instead. Plausibility check: only nitrogen that resists conversion to ammonium ion causes the method to fail, which matches the nitro, azo, and pyridine cases.