Aldol reaction is an important organic reaction of aldehydes and ketones. This reaction is shown by aldehydes and Ketones having alpha-hydrogen atom in the compound. Such a-hydrogen atom containing aldehydes and ketones will undergo self condensation in presence dilute alkalies like dil.NaOH (dilute Sodium hydroxide), dil.Ba(OH)2(dilute Barium hydroxide) forming alpha, β-hydroxyaldehyde (an aldol) or β-hydroxyketone (a Ketol) .The Aldol or the Ketol thus formed in the reaction upon heating eliminates a molecule of water (i.e. it undergoes dehydration) forming alpha, β-unsaturated Aldehydes or Ketones respectively. This reaction now is called Aldol condensation reaction.

Aldol condensation is the correct answer.

Explanation:
Aldol condensation is a reaction between two carbonyl compounds, such as aldehydes or ketones, to form a β-hydroxy carbonyl compound. In this reaction, a molecule of water is eliminated, and a new carbon-carbon bond is formed.

In the given reaction, ethanal (also known as acetaldehyde) reacts with an alkali to give 3-hydroxybutanal. Let's break down the reaction step by step:

Step 1: Formation of enolate ion
Ethanal (CH3CHO) reacts with an alkali (such as sodium hydroxide, NaOH) to form an enolate ion. The alkali acts as a base and abstracts a proton from the α-carbon of ethanal, resulting in the formation of an enolate ion. The enolate ion has a negative charge on the oxygen and a double bond between the carbon and oxygen.

CH3CHO + NaOH → CH3CH(OH)Na

Step 2: Attack of enolate ion on another molecule of ethanal
The enolate ion now acts as a nucleophile and attacks another molecule of ethanal. The nucleophilic carbon of the enolate ion attacks the carbonyl carbon of the second ethanal molecule, resulting in the formation of a new carbon-carbon bond.

CH3CH(OH)Na + CH3CHO → CH3CH(OH)CH(OH)CH3

Step 3: Elimination of water molecule
In the final step, a water molecule is eliminated from the β-hydroxy carbonyl compound formed in the previous step. This elimination of water leads to the formation of 3-hydroxybutanal.

CH3CH(OH)CH(OH)CH3 → CH3CH(OH)CH2CHO + H2O

Therefore, the given reaction between ethanal and alkali to give 3-hydroxybutanal is an example of aldol condensation.

The answer can be guessed from the product as it is possible through aldol condensation