Number of halides ion among $\left({\mathrm{F}}^{-},{\mathrm{Cl}}^{-},{\mathrm{Br}}^{-},{\mathrm{I}}^{-}\right)$ which change their o:odation number on heating with  ${\mathrm{MnO}}_2+\text{ conc. }{\mathrm{H}}_2{\mathrm{SO}}_4$.

Concentrated sulfuric acid isn't a strong enough oxidizing agent to oxidize fluoride or chloride ions.

The sulfuric acid is converted to sulfur dioxide gas by the bromide ions. This results in a drop in the sulfur's oxidation state from +6 in sulfuric acid to +4 in sulfur dioxide.

${\mathrm{H}}_2{\mathrm{SO}}_4+2{\mathrm{H}}^{+}+2{\mathrm{Br}}^{-}\to {\mathrm{Br}}_2+{\mathrm{SO}}_2+2{\mathrm{H}}_2\mathrm{O}$

The hydrogen sulfide is likely the most significant reduction product in this group. Its formation's half-equation is as follows:

${\mathrm{H}}_2{\mathrm{SO}}_4+8{\mathrm{H}}^{+}+8{\mathrm{I}}^{-}\to 4{\mathrm{I}}_2+{\mathrm{H}}_2\mathrm{S}+4{\mathrm{H}}_2\mathrm{O}$

There are 3 halide ions which changes their oxidation number on heating with  ${\mathrm{MnO}}_2+\text{ conc. }{\mathrm{H}}_2{\mathrm{SO}}_4$.