Assertion (A): Sulphanilic acid can exist as Zwitter-ion but not p-nitro benzoic acid. 

Reason (R): - $\mathrm{COOH}$ is more acidic than $-{\mathrm{SO}}_3\mathrm{H}$ group

Sulphanilic acid

Zwitter ion of sulphanilic acid

So, due to the presence of an amino group and sulfonic acid group, sulphanilic acid can form zwitter- ions.

For p-nitrobenzoic acid group, the lone of nitrogen is involved in resonance due to which the acidic character of $-{\mathrm{NH}}_2$ group decreases and these groups do not transfer and accept ${\mathrm{H}}^{+}$ions respectively.

Also, sulfonic acid is stronger than carboxylic acid. Thus, the assertion is true but the reason is false.

Hence, option (C) is correct.