To solve this problem, we need to determine which diazonium salt is the most stable.$\newline$Diazonium salts have the general formula $RN_2^+X^-,$ where $R$ is an organic group.$\newline$The stability of diazonium salts is influenced by the nature of the $R$ group.$\newline$Aromatic diazonium salts, such as those derived from benzene, are more stable than aliphatic diazonium salts.$\newline$This is because the positive charge on the nitrogen can be delocalized into the aromatic ring,$\newline$providing additional resonance stabilization.$\newline$Let's analyze the options:$\newline$• Option 1: $CH_3N_2^+X^-$ (methyl diazonium salt) is an aliphatic diazonium salt and is relatively unstable.$\newline$• Option 2: $C_6H_5N_2^+X^-$ (benzenediazonium salt) is an aromatic diazonium salt and is more stable due to resonance.$\newline$• Option 3: $CH_3CH_2N_2^+X^-$ (ethyl diazonium salt) is also an aliphatic diazonium salt and is unstable.$\newline$• Option 4: $C_6H_5CH_2N_2^+X^-$ (benzyl diazonium salt) is an aliphatic diazonium salt, despite having a benzyl group,$\newline$ and is less stable than aromatic diazonium salts.$\newline$Therefore, the most stable diazonium salt is the benzenediazonium salt, corresponding to Option 2.