Step 1: Recall the SNAr mechanism. Aromatic nucleophilic substitution proceeds by addition of the nucleophile to give a Meisenheimer intermediate, followed by elimination of the leaving group. Which step is slow (rate determining) depends on how stabilised that intermediate is. Step 2: Analyse reaction I (chlorobenzene). Plain chlorobenzene has no activating electron withdrawing group. The negatively charged intermediate is not stabilised, so the addition of $OH^-$ to the ring is hard and slow. Here the rate determining step is the attack of $OH^-$. Step 3: Analyse reaction II (p-nitrochlorobenzene). The para nitro group strongly withdraws electrons and stabilises the Meisenheimer intermediate, so the nucleophilic attack becomes fast. Now the slower step is the loss of chloride, i.e. cleavage of the $C-Cl$ bond. Step 4: Compare the two rate determining steps. For I it is attack of $OH^-$; for II it is $C-Cl$ bond cleavage. Step 5: Match to the options. We need attack of $OH^-$ in I and $C-Cl$ cleavage in II, which is option (3). Step 6: Conclude. \[ \boxed{\text{I: attack of }OH^-;\ \text{II: }C\text{-}Cl\text{ cleavage (Option 3)}} \]
