The correct order of reactivity of the following benzyl halides towards reaction with KCN is:

This reaction proceeds via the Sₙ1 mechanism.
The rate depends on the stability of the benzyl carbocation formed after departure of Br.

Key Idea:
Electron-donating groups increase carbocation stability and hence increase rate.
Electron-withdrawing groups decrease carbocation stability and hence decrease rate.
Resonance effects (para position) are stronger than inductive effects (meta position).

Analysis:

(b) p-NH₂
Strong +M effect, donates electron density by resonance, gives maximum stabilization → highest reactivity

(a) p-OH
Also shows +M effect but weaker than NH₂ → slightly lower reactivity

(d) m-NO₂
Shows -I effect only (no resonance at meta position) → moderate destabilization

(c) p-NO₂
Strong -M and -I effect, withdraws electron density strongly → least stable carbocation

Final Order:
b > a > d > c

Answer: p-NH₂ > p-OH > m-NO₂ > p-NO₂