What happens when formic acid reacts with acidified ${\mathrm{KMnO}}_4$ solution?

Arrange the following in the increasing order of acidic strength.

$\left(\mathrm{i}\right) \mathrm{Br}-{\mathrm{CH}}_2\mathrm{COOH}, \left(\mathrm{ii}\right) \mathrm{Cl}-{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{COOH} \left(\mathrm{iii}\right) \mathrm{F}-{\mathrm{CH}}_2\mathrm{COOH} \left(\mathrm{iv}\right) {\mathrm{CH}}_3\mathrm{COOH}$.

Complete the following equation by writing the structures of A, B and C.

${\mathrm{CH}}_3\mathrm{COOH} \stackrel{{\mathrm{Cl}}_2/\mathrm{P}}{\to } \mathrm{A} \stackrel{\mathrm{NaCN}}{\to } \mathrm{B} \underset{{\mathrm{H}}^{+}}{\overset{{\mathrm{H}}_2\mathrm{O}}{\to }} \mathrm{C}$