The correct stability order of the following resonance structures is:CH2=C=O ; H2C-C+=O  ;  H2O=C+-O ;  H2C-C=O          (I)                 (II)                          (III)                 (IV)

I is the most stable resonating structure because it is neutral. IV has complete octet on each atoms, II and III both have incomplete octet at 'C' atom bearing positive charge but in 'III' structure negative charge is present on more electronegative atom i.e. oxygen.