Hi,
Can you give me the relative stability of these carbocations with respect to each other :

Stability of  carbocation are based on degree of substitution (more alkyl groups = more stable due to electron repelling nature of alky groups ) and resonance. Here i is a simple primary but allylic system with resonance stabilisation by the p bonds. ii is also an allylic system, but more substituted and hence more resonance . iii is the very unstable methyl cation. So overall,
  ii > i > iii