The correct order of the following compounds showing increasing tendency towards nucleophilic substitution reaction is:

Step 1: Identify the compounds

From the image, the four chlorobenzenes are:

• (i) Chlorobenzene (plain)
• (ii) p-Nitrochlorobenzene (one $\ce{NO2}$ at para)
• (iii) 2,4-Dinitrochlorobenzene (two $\ce{NO2}$ groups)
• (iv) m-Nitrochlorobenzene ($\ce{NO2}$ at meta)

Step 2: Understand $S_NAr$ mechanism

Nucleophilic aromatic substitution requires:

• Electron-withdrawing groups (EWG)
• EWG at ortho or para positions (resonance stabilization)

Step 3: Rank by reactivity

Factors:

1. Number of EWG: More = faster
2. Position of EWG: ortho/para > meta
3. Strength of EWG: $\ce{NO2}$ is very strong

Analysis:

(iii) 2,4-Dinitrochlorobenzene:

• Two $\ce{NO2}$ groups (both at activating positions)
• Highest reactivity ✓✓✓

(ii) p-Nitrochlorobenzene:

• One $\ce{NO2}$ at para
• Can stabilize Meisenheimer complex
• High reactivity ✓✓

(i) Chlorobenzene:

• No EWG
• Very slow or no reaction
• Low reactivity ✓

(iv) m-Nitrochlorobenzene:

• One $\ce{NO2}$ at meta
• Cannot stabilize through resonance
• Only inductive effect (weak)
• Lowest reactivity ✗

Increasing order: (iv) < (i) < (ii) < (iii)

Answer: (d)

Key Points:

$S_NAr$ activation:

• ortho/para $\ce{NO2}$: Strong activation (resonance + inductive)
• meta $\ce{NO2}$: Weak activation (inductive only)
• No EWG: Very slow

Meisenheimer complex:

• Anionic intermediate in $S_NAr$
• Stabilized by EWG through resonance
• ortho/para EWG can delocalize negative charge
• meta EWG cannot (wrong position for resonance)

Reactivity order: 2,4-Dinitro > ortho-Nitro ≈ para-Nitro > meta-Nitro > No activation