In aqueous solution the pKa values of the conjugate acids are: aniline 4.6, 3-nitroaniline 2.5, 4-nitroaniline 1.0, 2-nitroaniline $-0.3$. The weakest base in aqueous solution is

A lower pKa of the conjugate acid means a weaker base. 2-Nitroaniline has the lowest value ($-0.3$). The ortho nitro group withdraws electrons strongly by $-I$ and $-M$ at close range, and the $\mathrm{N}-\mathrm{H}$ hydrogen-bonds to the nitro oxygen, which holds back the lone pair.