The structural feature in carbonyl compound for Aldol condensation

The correct option is presence of at least one $\alpha$-hydrogen.

Detailed Explanation:
Aldol condensation is a characteristic reaction of aldehydes and ketones containing at least one $\alpha$-hydrogen (a hydrogen atom attached to the carbon adjacent to the carbonyl group). The reaction typically proceeds via the following steps:

1. Formation of an Enolate Ion:
A dilute base abstracts an acidic $\alpha$-hydrogen from the carbonyl compound, forming a resonance-stabilized enolate nucleophile. This step is only possible if there is at least one $\alpha$-hydrogen present. Without an $\alpha$-hydrogen, the enolate ion cannot form.

2. Nucleophilic Attack:
The enolate ion then attacks the electrophilic carbonyl carbon of another molecule of the aldehyde or ketone, forming a $\beta$-hydroxy carbonyl compound (known as an aldol).

3. Dehydration:
Upon heating, this $\beta$-hydroxy carbonyl compound undergoes dehydration to yield an $\alpha$,$\beta$-unsaturated carbonyl compound.

Therefore, the presence of at least one $\alpha$-hydrogen is the essential structural feature required for a carbonyl compound to undergo Aldol condensation.