The correct options are
A (c) is the least stable structure
C (a) is the most stable structure
Structure (a) has the maximum number of covalent bonds and least number of charges , so it is the most stable.
Among (b),(c),(d) since all of them have positive charges on nitrogen, on comparing oxygen and carbon.
Structure (b) has two negative charges on electronegative atom oxygen, so it is more stable than (c) & (d).
Amog, (c),(d) like charges are closer to each other in (c) as compared to (d).
So (c) is the least stable structure.