Aldol condensation results due to-

The correct answer is Option (1) → Acidity of α-hydrogen in aldehydes and ketones

The α-hydrogens of aldehydes and ketones are acidic because of the strong electron-withdrawing effect of the carbonyl group and because the conjugate base (the enolate ion) is stabilised by resonance. Dilute alkali removes an α-hydrogen, and the carbanion adds to the C=O group of a second molecule, giving a β-hydroxy aldehyde (aldol) or ketone (ketol). On heating this loses water: aldol condensation.

    \( \displaystyle \ce{2CH3CHO} \xrightarrow{\text{dil. }\ce{NaOH}} \ce{CH3CH(OH)CH2CHO} \xrightarrow[-\ce{H2O}]{\Delta} \ce{CH3CH=CHCHO} \)

Why the other options are incorrect

• Option (2) Acidity of α-hydrogen in carboxylic acids — Incorrect. In a carboxylic acid the −COOH proton is far more acidic, so base forms the carboxylate ion; carboxylic acids do not undergo aldol condensation.

• Option (3) Acidity of α-hydrogen in amines — Incorrect. Amines have no carbonyl group to make an α-hydrogen acidic; they are bases.

• Option (4) Acidity of α-hydrogen in Haloalkanes — Incorrect. In haloalkanes a base removes a β-hydrogen together with \( \ce{X^{-}} \) (elimination) or substitutes the halogen; there is no carbonyl group for an aldol reaction.

NCERT (2026-27 edition):
Reactions due to α-Hydrogen – “Acidity of α-hydrogens of aldehydes and ketones: The aldehydes and ketones undergo a number of reactions due to the acidic nature of α-hydrogen. The acidity of α-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base. …
(i) Aldol condensation: Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction. The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products. The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation.”