Step 1: Understanding the Concept
Basicity depends on the availability of the lone pair on nitrogen.
In aniline the lone pair is partly delocalised into the benzene ring, so it is a weak base.
Step 2: Compare with the others
\(\text{NH}_3\) and \(\text{CH}_3\text{NH}_2\) have a localised lone pair (and \(\text{CH}_3\) adds +I effect), so they are stronger bases than aniline.
N-methylaniline is slightly more basic because of the +I effect of the methyl group.
Step 3: The weaker base
In p-nitroaniline, the \(-\text{NO}_2\) group withdraws electrons by -M and -I effects.
This pulls the lone pair further from nitrogen, so p-nitroaniline is less basic than aniline.
Final Answer:
p-Nitroaniline is less basic than aniline.
\[ \boxed{\text{(A) }\text{p-nitroaniline}} \]