Assertion (A): Aniline is basic in nature 

Reason (R): Aniline reacts with $\mathrm{HCl}$  and forms a salt anilinium chloride

Aniline is basic in nature with $\mathrm{pKa}=4.6$

Since the lone pair of nitrogen in aniline is partially delocalized, it is available to react with a strong acid like $\mathrm{HCl}$ to form a salt known as anilinium chloride.

The reaction takes place is as follows,

Thus, both the assertion and reason are true but the reason is not a correct explanation for the assertion.

Hence, option (B) is correct.