Compare the stability of resonating structures :

The correct option is I > II > III.

To determine the relative stability of the resonating structures shown in the image, we apply the following rules of resonance in order of priority:

1. Neutral vs. Charged Structures:
Resonance structures that are neutral (without any charge separation) are more stable than those containing formal positive and negative charges because separating charges requires energy.
In the provided image, Structure (I) (${\mathrm{CH}}_3-C(=O)-\mathrm{OH}$) is completely neutral with no formal charges. Therefore, Structure (I) is the most stable structure.

2. Completeness of Octets:
Resonance structures where all atoms have complete valence shell octets are significantly more stable than those with incomplete octets.
Let us evaluate the octets for the charged structures (II) and (III) (labeled as the middle and rightmost structures in the image, respectively):

• Structure (II) (${\mathrm{CH}}_3-C\left(O^{-}\right)=O^{+}H$): The carbon atom forms 4 covalent bonds (8 valence electrons). The negatively charged oxygen atom ($O^{-}$) has 1 bond and 3 lone pairs (8 valence electrons). The positively charged oxygen atom ($O^{+}$) has 3 bonds and 1 lone pair (8 valence electrons). Since all atoms have complete octets, this structure is relatively stable despite the charge separation.
• Structure (III) (${\mathrm{CH}}_3-C^{+}\left(\mathrm{OH}\right)-O^{-}$): The central carbon atom has a formal positive charge and forms only 3 single bonds with no lone pairs. This leaves it with only 6 valence electrons, resulting in an incomplete octet.

Since Structure (II) has complete octets for all atoms while Structure (III) has an incomplete octet on carbon, Structure (II) is more stable than Structure (III).

Combining these points, the overall stability order is:

$I>\mathrm{II}>\mathrm{III}$