The correct order of stability of the carbocations (I) $p\text{-}\mathrm{CH}{}_3\mathrm{O}-\mathrm{C}{}_6\mathrm{H}{}_4-\mathrm{CH}{}_2{}^{+}$, (II) $p\text{-}\mathrm{CH}{}_3-\mathrm{C}{}_6\mathrm{H}{}_4-\mathrm{CH}{}_2{}^{+}$, (III) $\mathrm{C}{}_6\mathrm{H}{}_5-\mathrm{CH}{}_2{}^{+}$ and (IV) $p\text{-}\mathrm{O}{}_2\mathrm{N}-\mathrm{C}{}_6\mathrm{H}{}_4-\mathrm{CH}{}_2{}^{+}$ is

All are benzyl cations stabilised by resonance. The p-methoxy group donates strongly by +M; the p-methyl group donates by +I and hyperconjugation; the unsubstituted ring has none; the p-nitro group withdraws by -M and -I and destabilises the cation. Order: I > II > III > IV.