Which of the following is least basic than aniline?

The basicity of amines depends on the availability of the lone pair of electrons on the nitrogen atom for protonation.

In aniline, the lone pair of electrons on the nitrogen atom is delocalized over the benzene ring through resonance, making it less basic than aliphatic amines like $\text{CH}_3\text{NH}_2$ and ammonia ($\text{NH}_3$), where the lone pair is localized.

In N-methyl aniline, the electron-donating methyl group exerts a +I effect, which increases the electron density on the nitrogen atom, making it more basic than aniline.

In p-nitroaniline, the $-\text{NO}_2$ group is strongly electron-withdrawing due to its -I and -R effects. This significantly decreases the electron density on the nitrogen atom, making the lone pair even less available for protonation compared to aniline.

Therefore, p-nitroaniline is less basic than aniline.

Answer: p-nitroaniline