$C _{6} H _{5} CH _{2} N _{2}^{+} X ^{-}, CH _{3} N _{2}^{+} X^{-}$ and $CH _{3} CH _{2} N _{2}^{+} X^{-}$ are primary amine diazonium salts which are readily decomposed and liberate $N _{2}$ gas but $C _{6} H _{5} N _{2}^{+} X ^{-}$ is an aryl diazonium salt which is
stabilised by resonance of benzene ring which is directly attached with nitrogen.