Correct stability order of resonance structures
Compound II and III have 2 aromatic rings whereas I and IV have only 1 aromatic ring, and more the number of aromatic rings, the more the compound is stable.


  Aromatic molecules are very stable, and do not break apart easily to
  react with other substances. Organic compounds that are not aromatic
  are classified as aliphatic compounds—they might be cyclic, but only
  aromatic rings have special stability (low reactivity).


Thus stability of II= stability of III and stability of I = stability of IV.

And since II and III have greater number of aromatic rings than I and IV, 

Stability(II,III) > Stability(I,IV)      

Thus the answer would be [Option D] and not [Option C].