Keeping with the idea that the more stable structure is formed more readily, we need to compare the stability of the enolates leading to D and E . Both have two significant resonance structures. However, the ester oxygen that is not part of the carbonyl group donates electron density to the enolate by resonance. This electron donation increases the magnitude of the negative charge, thus destabilizing the enolate. This dehydration step is driven by the formation of a conjugated system.
Hydroxide ion is normally a poor leaving group.
In reaction I , hydroxide departure forms a conjugated enone, so we expect reaction I to proceed with a nonzero reaction rate. The ease of dehydration is a function of the energy of activation of this step. The energy of activation is controlled by the transition state. If the transition state is more stable, the energy of activation is lower, and the step proceeds more readily. The dehydration step leads to a conjugated product.