${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{COOH}\left(\mathrm{X}\right)\underset{1\mathrm{mole}}{\overset{{\mathrm{Br}}_2/\mathrm{Red}\mathrm{P}}{\to }}\mathrm{A}\stackrel{\mathrm{alc}.\mathrm{KOH}}{\to }\mathrm{B}\stackrel{\mathrm{HBr}}{\to }\mathrm{C}.$     Most acidic compound in this conversion is

$\mathrm{A}={\mathrm{CH}}_3-\underset{\begin{array}{l}|\\ {\mathrm{Br}}^{ }\end{array}}{\mathrm{CH}}-\mathrm{COOH};\mathrm{B}={\mathrm{CH}}_2=\mathrm{CH}-\mathrm{COOH};$

$\mathrm{B} \to \mathrm{C}$

$$\begin{gathered} {\mathrm{CH}}_2=\mathrm{CH}-\mathrm{COOH}\stackrel{{\mathrm{H}}^{+}}{\to }\underset{\mathrm{Less}\mathrm{Stable}}{{\mathrm{CH}}_3-\stackrel{+}{\mathrm{CH}}-\mathrm{COOH}}+\underset{\mathrm{More}\mathrm{Stable}}{\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2-{\mathrm{CH}}_2-\mathrm{COOH}} \\ -\mathrm{COOH} \mathrm{group} \mathrm{shows} \mathrm{negative} \mathrm{inductive} \mathrm{effect} \mathrm{and} \mathrm{destabilizes} \mathrm{the} \mathrm{positive} \mathrm{charge} \\ \mathrm{As} \mathrm{distance} \mathrm{increases} \mathrm{stability} \mathrm{increases} \end{gathered}$$

$\begin{array}{l} \\ \stackrel{+}{\mathrm{C}}{\mathrm{H}}_2-{\mathrm{CH}}_2-\mathrm{COOH}\stackrel{{\mathrm{Br}}^{-}}{\to }\underset{\begin{array}{l}|\\ {\mathrm{Br}}^{ }\end{array}}{{\mathrm{CH}}_2}-{\mathrm{CH}}_2-\mathrm{COOH}\left(\mathrm{C}\right)\end{array}$

Acidic strength of

$\mathrm{\alpha }$–halo carboxylic acid > HCOOH > $\mathrm{\beta }$–halo Carboxylic acid > ${{\mathrm{C}}_6\mathrm{H}}_5\mathrm{COOH}$> ${\mathrm{CH}}_2=\mathrm{CH}-\mathrm{COOH}$ > ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_2\mathrm{COOH}$ > ${\mathrm{CH}}_3\mathrm{COOH}$ >${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{COOH}$

$\therefore$A  >   C   >    B  >   X