Which of the following carbocations are stabilised by resonance?

Allyl and benzyl cations have an empty p orbital overlapping with a pi system. In $\mathrm{CH}{}_3-\mathrm{O}-\mathrm{CH}{}_2{}^{+}$ an oxygen lone pair overlaps with the empty p orbital, giving $\mathrm{CH}{}_3-\mathrm{O}{}^{+}=\mathrm{CH}{}_2$. In the phenyl cation the empty orbital is an sp2 orbital in the plane of the ring, orthogonal to the pi system, so there is no resonance.