Explanation

(b)
$PCl_{3},PCl_{5}\ and\ SOCl_{2}$ are used to replace –OH group of an alcohol or an acid by –Cl group
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When acid reacts with $Cl_{2}$ in presence of red phosphorus, $\propto$-chloro acid is obtained. (Hell-Vohlard-Zelinsky reaction).
$\ \ \ \ Cl$
$\ \ \ \ \ |$
$CH_{3}CH_{2}COOH + Cl_{2}\ \underset{\mathrm{\Delta}}{\overset{\ \ \ Red\ P\ \ \ }{\rightarrow}}CH_{3}CHCOOH + HCl$
$\propto$-chloropropionic acid