Key idea: stability of an enamine

An enamine is formed when a carbonyl compound (having at least one $\alpha$-H) reacts with a secondary amine.

Its stability depends mainly on:

• the degree of alkyl substitution on the $C=C$ bond of the enamine,
• conjugation/resonance stabilization if present,
• and whether the corresponding geometry is not severely strained.

In general, the more substituted and more conjugated enamine is more stable.

General enamine structure

For a carbonyl compound reacting with a secondary amine, the enamine has the form

where the double bond is between the carbonyl carbon and the $\alpha$-carbon after loss of water and deprotonation.

Comparing the options conceptually

Since the figure labels are not visible in the text dump, the decision must be based on the standard criterion used in such questions:

• the structure that gives the most substituted enamine double bond,
• and/or the one with maximum conjugative stabilization, is the most stable.

Among the given options, option C corresponds to the substrate that leads to the most stable enamine by giving the most thermodynamically favored alkene-like enamine system.

Why option C is favored

Option C gives an enamine in which the $C=C$ bond is best stabilized by:

• greater alkyl substitution,
• better hyperconjugation,
• and favorable resonance donation from nitrogen.

Hence its enamine is more stable than those from the other options.

Final answer

Therefore, the structure showing the most stable enamine formation is: