Which of the following is the strongest Bronsted base?

Bronsted base strength = ability to accept a proton = availability of lone pair on N.

Step 1 — Analyse each compound:

(1) Aniline ($\mathrm{C_6H_5NH_2}$): Lone pair on N is delocalized into the benzene ring (resonance) $\rightarrow$ reduced basicity. $\mathrm{pK_b} \approx 9.4$

(2) Diphenylamine ($(\mathrm{C_6H_5})_2NH$): Two phenyl groups withdraw lone pair $\rightarrow$ even weaker base than aniline. $\mathrm{pK_b} \approx 13.1$

(3) Pyrrole: N lone pair is part of the aromatic $\pi$ system (6 $\pi$ electrons) $\rightarrow$ essentially non-basic. $\mathrm{pK_b} \approx 13.6$

(4) Pyrrolidine (cyclic secondary amine): No aromatic ring, no resonance withdrawal $\rightarrow$ lone pair fully available $\rightarrow$ strongest base. $\mathrm{pK_b} \approx 2.7$

Order of basicity: Pyrrolidine >> Aniline > Diphenylamine > Pyrrole

Answer: Option (4) — Pyrrolidine