Step 1: Understand what affects nucleophilic substitution reactivity. Nucleophilic substitution (SN) reactions require the carbon bearing the halogen to be electron-poor enough for the nucleophile to attack. The ease of C-X bond breaking determines reactivity. Step 2: Recall the effect of the benzene ring on the C-X bond. In aryl halides, the halogen is directly attached to the sp2-hybridised carbon of the benzene ring. The lone pairs of the halogen participate in resonance with the pi-system of the ring, giving the C-X bond partial double-bond character. Step 3: Resonance in aryl halides makes the carbon electron-rich. Due to resonance donation of the halogen lone pair into the ring, the carbon bearing the halogen becomes electron-rich (not electron-poor). This strongly opposes nucleophilic attack, which requires an electron-deficient carbon. Step 4: Why aryl halides are least reactive in SN reactions. Both electronic and steric factors work against SN in aryl halides: (i) the carbon bearing the halogen is electron-rich due to resonance, (ii) the C-X bond has partial double-bond character making it harder to break, (iii) the flat aromatic ring blocks backside attack needed for SN2. Step 5: Compare reactivity order of different halide types. Reactivity in SN: allylic and benzylic > alkyl > vinyl and aryl. Vinyl and aryl halides are least reactive because of resonance stabilisation of the C-X bond. Alkyl halides react readily by SN1 or SN2. Step 6: Identify Fig B as the least reactive. Fig B shows an aryl halide (halogen directly on the benzene ring). It is least reactive towards nucleophilic substitution due to resonance strengthening of the C-X bond and the electron-rich nature of the ipso carbon. \[ \boxed{\text{Fig B (aryl halide) is least reactive towards nucleophilic substitution}} \]
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