Which of the following reduction are actually employed in commercial extraction of metals?

Commercial chromium extraction involves aluminum reduction. The reaction is having follows

${\mathrm{Cr}}_2{\mathrm{O}}_3+2\mathrm{Al}\to 2\mathrm{Cr}+{\mathrm{Al}}_2{\mathrm{O}}_3$

By heating it in the presence of a diluted solution of  $\mathrm{NaCN}$ and ${\mathrm{O}}_2$ ${\mathrm{Au}}_2\mathrm{S}$ is leached

The following reaction will obtain

${\mathrm{Au}}_2\mathrm{S}+4\mathrm{NaCN}\rightleftharpoons 4{\mathrm{Na}}^{+}2{\left[\mathrm{Au}(\mathrm{CN}{)}_2\right]}^{2-}+{\mathrm{S}}^{2-}$

Gold is produced by a displacement process in which the cyanide molecule is treated with zinc

The reaction is having follows,

$2\mathrm{Na}\left[\mathrm{Au}(\mathrm{CN}{)}_2\right]+\mathrm{Zn}\to {\left[\mathrm{Zn}(\mathrm{CN}{)}_4\right]}^{2-}+2\mathrm{Au}\downarrow$

Hence, options B C and D are correct.