The structural feature in carbonyl compound for Aldol condensation

presence of at least one $α$-hydrogen

The correct answer is Option (2) → presence of at least one $α$-hydrogen

• Aldol condensation occurs in aldehydes or ketones that have at least one α-hydrogen (hydrogen on the carbon adjacent to the carbonyl group).
• The α-hydrogen is required for enolate ion formation, which is the key step in the aldol reaction.

Why others are incorrect:

• β-hydrogen → not relevant 
• Concentrated base → a condition, not a structural feature 
• Lack of α-hydrogen → such compounds do not undergo aldol condensation 

 Hence, the correct option is presence of at least one α-hydrogen.