Explanation

(b)
Aldol condensation aldehydes containing $\alpha -$hydrogen undergo self addition in presence of a base to form products called ‘aldols’. The reaction is called ‘aldol condensation’.
Example Two molecules of acetaldehyde combine with each other in presence of dil. NaOH to form 3-hydroxybutanal.
$OH$
|
$CH_{3}CHO + HCH_{2}CHO\overset{\ \ \ \ OH^{-}\ \ \ }{\rightarrow}\ CH_{3} - CH - CH_{2} - CHO$
3-hydroxybutanal
(aldol)