Explanation

(b)
When acetaldehyde is treated with aqueous sodium hydroxide solution, it sundergoes aldol condensation (because of the presence of $\alpha -$H atom) as.
$\begin{matrix}
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\
CH_{3}CHO + HCH_{2}CHO\ \underset{NaOH}{\overset{\ \ \ Dil.aqueous\ \ \ \ }{\rightarrow}}CH_{3}CH
\end{matrix}$
$*$
$= CHCHO\ \overset{\ \ \ \ \mathrm{\Delta}\ \ \ \ }{\leftarrow}\ \ CH_{3}CH(OH)CH_{2}CHO$
(shows geometrical isomerism)
aldol
(shows optical isomerism)