In winter, special type of clouds called polar stratospheric clouds are formed over Antarctica.These polar stratospheric clouds provide surface on which chlorine nitrate formed gets hydrolysed to form hypochlorous acid.It also reacts with hydrogen chloride produced to give molecular chlorine

The reactions are:

${\mathrm{ClONO}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)\to \mathrm{HOCl}\left(\mathrm{g}\right)+{\mathrm{HNO}}_3\left(\mathrm{g}\right)$

${\mathrm{ClONO}}_2\left(\mathrm{g}\right)+\mathrm{HCl}\left(\mathrm{g}\right)\to {\mathrm{Cl}}_2\left(\mathrm{g}\right)+{\mathrm{HNO}}_3\left(\mathrm{g}\right)$

Hence, A, B and C are $\mathrm{HOCl},{\mathrm{HNO}}_3,{\mathrm{Cl}}_2$