Assertion (A): Aniline is more stable than anilinium ion 

Reason $\left(\mathrm{R}\right)$:  Aniline is more resonance stabilised than anilinium ion

For aniline,

For anilinium ion,

So, anuline has 5 resonating structures while anilinium ion has only 3 resonating structures. We know that, Stability of a molecule $\alpha$ Number of resonating structures That means aniline is more stable than anilinium ion due to more resonating structures. Also, the above resonance structures of anilinium ions are hypothetical and not possible because charge dispersion at the ring may involve pentavalent nitrogen structure which is very unstable.

Therefore, aniline is more resonance stabilised than anilinium ion.

Thus, both assertion and reason are true and reason is the correct explanation of assertion.

Hence, option (A) is correct.