which structure is not possible?

Brief Explanations:

In organic chemistry, carbon typically forms 4 bonds. We need to check each structure for compliance with the octet - rule and the typical bonding patterns of carbon, oxygen and other atoms present. Analyze the valency of each atom in the structures.

Answer:

(Without labels on the structures, it's not possible to directly point out which one. But the general approach is to look for atoms with incorrect valencies. For example, if a carbon has 5 bonds or an oxygen has 3 single - bonds without formal charges to account for it, that structure would be incorrect.)