Please explain 
1. The correct order is as follows:
(iii) > (iv) > (v) > (vii) > (ii) > (i) > (vi)
2. The stability of (iii), (iv) and (v) is accounted by the formation of diad, in which all the atoms have complete octet and stability is enhanced even if O or N have positive charge on it. This is as shown:
3. However, when oxygen and nitrogen are connected at 3 position from carbocationic carbon, they have their electronegative effects enhanced and thus, O being more electronegative makes molecule more unstable as compared to N.
4. Finally, NO2 is always electron-withdrawing in nature and makes electron deficient carbocation even more unstable.
(iii) > (iv) > (v) > (vii) > (ii) > (i) > (vi)
2. The stability of (iii), (iv) and (v) is accounted by the formation of diad, in which all the atoms have complete octet and stability is enhanced even if O or N have positive charge on it. This is as shown:
3. However, when oxygen and nitrogen are connected at 3 position from carbocationic carbon, they have their electronegative effects enhanced and thus, O being more electronegative makes molecule more unstable as compared to N.
4. Finally, NO2 is always electron-withdrawing in nature and makes electron deficient carbocation even more unstable.