A solution is 0.1M with respect to KCl and 0.2M with respect to ${\mathrm{MgCl}}_2$. The molarity of $C{l}^{-}$ ions in the solution is

Molarity of KCl = 0.1M

KCl gives $K^{+} C{l}^{-} ions$

0.1M KCl gives 0.1M of $C{l}^{-} ions$

Molarity of ${\mathrm{MgCl}}_2$ = 0.2M

${\mathrm{MgCl}}_2$ gives $1 M{g}^{+} and 2 C{l}^{-} ions$

0.2M ${\mathrm{MgCl}}_2$ gives $0.2\times 2=0.4M$ of $C{l}^{-} ions$

Total molarity of $C{l}^{-} ions$ = 0.1 + 0.4 = 0.5M

Therefore, the required answer is D) 0.5 M