Most acidic substance is 

$$\begin{gathered} {\mathrm{CH}}_3{\mathrm{COCH}}_2{\mathrm{CH}}_3\underset{{\mathrm{H}}^{+}}{\overset{\mathrm{NaOI}}{\to }} {\mathrm{CHI}}_3+\underset{\mathrm{A}}{{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{COOH}} \\ {\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{COOH}\underset{1\mathrm{mole}}{\overset{{\mathrm{Br}}_2/\mathrm{red} \mathrm{P}}{\to }} \underset{\mathrm{B}}{{\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{Br}\right)\mathrm{COOH}} \\ {\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{Br}\right)\mathrm{COOH} \stackrel{\mathrm{alc}.\mathrm{KOH}}{\to } \underset{\mathrm{C}}{{\mathrm{CH}}_2=\mathrm{CH}-\mathrm{COOH}} \\ {\mathrm{CH}}_2=\mathrm{CH}-\mathrm{COOH}+\mathrm{HCl}\to \underset{\mathrm{D}}{{\mathrm{CH}}_2\left(\mathrm{Cl}\right)-{\mathrm{CH}}_2-\mathrm{COOH}} \end{gathered}$$

B is more acidic due to $\mathrm{\alpha }-\mathrm{Br}$atom.