The reaction of amines with mineral acids to form ammonium salts shows that these are basic in nature. Amines have an unshared pair of electrons on the nitrogen atoms due to which they behave as Lewis bases. The basicity of amines is related to their structure. The basic character of an amine depends upon the ease of formation of a cation by accepting a proton from the acid. The more stable the cation is relative to the amine, the more basic the amine.

Among the following which one has the highest pK_b value?

C₆H₅NH₂

Higher the value of pK_b, the weaker the basic strength. In aniline (C₆H₅NH₂), due to the delocalization of lone pairs, its availability for protonation is minimal. Hence, it is the least basic among all other amines.