The order of stability of the following carbocations:

(III) Benzyl Carbocation ($$C_6H_5-\overset{+}{C}H_2$$)

The positive charge on the carbon atom is in conjugation with the $$\pi$$-electron system of the benzene ring. The charge is delocalized to the ortho and para positions of the ring. It has strong $$+M$$ (Resonance). It has 4 resonating structures in total. This extensive delocalization makes it highly stable.

(I) Allyl Carbocation ($$CH_2=CH-\overset{+}{C}H_2$$)

The positive charge is in conjugation with a single carbon-carbon double bond. It also has $$+M$$ (Resonance) and has 2 resonating structures.

While it is resonance-stabilized, the degree of delocalization is lower than that in the benzyl carbocation.

(II) n-Propyl Carbocation ($$CH_3-CH_2-\overset{+}{C}H_2$$)

This is a primary ($$1^\circ$$) alkyl carbocation. There is no resonance possible here. Hyperconjugation and $$+I$$ effects are present here from the ethyl group. It has 2 $$\alpha$$-hydrogens. Since resonance is a much stronger stabilizing factor than hyperconjugation, this is the least stable carbocation among the three.

Stability: Benzyl (III) > Allyl (I) > n-Propyl (II)