(i) `(I)` is most stable, since it is stabilised by resonance and has six `alpha-H` atoms (hyperconjugation).
(ii) `(III)` is less stable than `(I)` but more stable than `(II)` and `(IV)`, since it is also stabilised by resonance and has three `alpha-H` atoms (hyperconjugtion).
(iii) `(II)` is more stable than `(IV)` and is stabilished only be hyperconjugative structure (five `alpha-H` atoms).
(IV) is least stable, it has only two `alpha`-H atoms.
the stability order is : `I gt III gt II gt IV`.
