Here option D is not correct as oxygen is evolved only when ${\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$ is heated with conc. ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ .

$2{\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7+8{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\stackrel{\mathrm{\Delta }}{\to }2{\mathrm{K}}_2\mathrm{S}{\mathrm{O}}_4+2\mathrm{C}{\mathrm{r}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3+8{\mathrm{H}}_2\mathrm{O}+3{\mathrm{O}}_2$

A. $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}+14{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\to 2\mathrm{C}{\mathrm{r}}^{3+}+7{\mathrm{H}}_2\mathrm{O}$

${\mathrm{S}}^{2-}\to \mathrm{S}+2{\mathrm{e}}^{-}\times 3$

B. $2{\mathrm{I}}^{-}\to {\mathrm{I}}_2+2{\mathrm{e}}^{-}$

C. $2\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{C}{\mathrm{l}}_2+2{\mathrm{e}}^{-}$