(A) $\rightarrow$ (B) Free $H ^{+}$ions are replaced by $Na ^{\oplus}$ which decreases conductance.
(B) $\rightarrow$ (C) Un-dissociated benzoic acid reacts with $NaOH$ and forms salt which increases ions & conductance increases.
(C) $\rightarrow$ (D) After equivalence point at (3), $NaOH$ added further increases $Na ^{\oplus} \& OH ^{\odot}$ ions which further increases the conductance.
