Which of the following will be the most stable diazonium salt $R{N}_2^{+}{X}^{-}$?

Aromatic diazonium salts (${\text{C}}_6{\text{H}}_5{N}_2^{+}{X}^{-}$) are more stable than aliphatic ones because the positive charge on the nitrogen can be dispersed through resonance into the benzene ring. Aliphatic diazonium salts are extremely unstable and decompose even at low temperatures.