Which of the following is the strongest base in aqueous solution?

Each additional phenyl on N, or a strongly withdrawing $-\mathrm{NO}{}_2$ group at the para position, further delocalises or withdraws the lone pair. Aniline (pKaH 4.6) is stronger than 4-nitroaniline (about 1.0), diphenylamine (about 0.8) and triphenylamine (very weak).