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, NOis always electron-withdrawing in nature and makes electron deficient carbocation even more unstable.

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