Step 1: Relate pKb to base strength. $pK_b = -\log K_b$. Lower $pK_b$ means larger $K_b$, which means a stronger base. The strongest base has the most available lone pair on nitrogen. Step 2: Structural factors affecting basicity of amines. Resonance delocalization of the lone pair into an aromatic ring (as in aniline) reduces basicity sharply. Electron-donating alkyl groups increase basicity. $sp^3$ nitrogen is more basic than $sp^2$ nitrogen. Electron-withdrawing substituents reduce basicity. Step 3: Figure A has the lowest pKb. Among the four structures, Figure A has the most electron-rich, non-delocalized lone pair on nitrogen (e.g., aliphatic or alkyl-substituted amine with no resonance loss), making it the strongest base and giving the lowest $pK_b$. \[ \boxed{\text{Figure A has the lowest } pK_b} \]
