(c)
It is a case of simultaneous solubility of salts with a common ion. Here solubility product of $CuCl\$is much greater than that of $AgCl$, it can be assumed that $Cl^{-}$ in solution comes mainly from $CuCl$.
$\Rightarrow$ $\left\lbrack Cl^{-} \right\rbrack = \sqrt{K_{sp}(CuCl)} = 10^{- 3}\ M$
Now for $AgCl\ :K_{sp} = 1.6 \times 10^{- 10}$
$\ = \left\lbrack Ag^{+} \right\rbrack\lbrack Cl^{-}\rbrack$
$= \left\lbrack Ag^{+} \right\rbrack \times 10^{- 3}$
$\Rightarrow$ $\left\lbrack Ag^{+} \right\rbrack = 1.6 \times 10^{- 7}$