The oxidation number of ${\mathrm{Cl}}^{-}$in ${\mathrm{CaOCl}}_2$ (bleaching powder) is:

Calcium hypochlorite is the chemical name for bleaching powder, which has the chemical formula ${\mathrm{CaOCl}}_2$. The oxidation states of the two chlorine atoms differ. The structure of the bleaching order is Cl−Ca−O−Cl.
The two chlorine atoms in the bleaching order structure are in distinct oxidation states because one chlorine atom is connected to calcium and the other chlorine atom is attached to an oxygen atom.
Regarding the question, the chlorine atom bonded to the calcium atom has an oxidation number of -1, while the oxidation number of all other chlorine atoms is +1.

Hence, option A is correct.