4-Nitroaniline (pKaH 1.0) is a weaker base than 3-nitroaniline (pKaH 2.5) mainly because

1. Athe nitro group is a donor at the para position
2. Bthe amino group in 4-nitroaniline is sp${}^3$ hybridised
3. Cthe nitro group at the para position withdraws electrons from the amino N by the $-M$ effect (in addition to $-I$), while at the meta position only the $-I$ effect operates
4. Dthe meta isomer forms stronger hydrogen bonds with the solvent through the nitro group