Assertion (A): Benzene diazonium salts are stable 

Reason (R): Benzene diazonium ion is stabilised by resonance

 ${\mathrm{RNH}}_2\stackrel{{\mathrm{NaNO}}_2+\mathrm{HCl}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{N}}^{2+}{\mathrm{Cl}}^{-}$

The above reaction is known as the diazotization reaction which gives benzene diazonium salts as the final product.

Resonance in benzene diazonium salt,

So, benzene diazonium salt is stabilised by resonance.

Hence, option (A) is correct.