Answer: B
The amino group at the para position donates electrons by +M effect, which destabilises the benzoate ion (pKa about 4.85). Each nitro-substituted acid has the anion stabilised by -I (and -M at para), giving pKa values of 3.5 or lower (3,5-dinitro about 2.8). So 4-aminobenzoic acid is the weakest.