Which of the following will be most stable diazonium salt R N 2 + X - ?

Benzene diazonium chloride is most stable due to conjugation.

Among the given diazonium salts, benzene diazonium halide${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{N}}_2^{+}{\mathrm{X}}^{-}$ is most stable due to conjugation of the N,N triple bond with benzene ring.

Aromatic diazonium salts such that Benzene Diazonium Halide are more stable than aliphatic diazonium salts due to dispersion of positive charge over the benzene ring caused by resonance. This is not found in aliphatic diazonium salts.