Eq.mass of copper chloride =99(given)
Eq.mass of chlorine=35.5(given)
$\therefore$ Eq.mass of copper chloride =Eq.mass of copper + Eq.mass of chlorine
$\therefore$ 99=E of Cu +35.5
$\therefore$ E of Cu=99-35.5=63.5
$\therefore$ Valency of copper =$\large\frac{\text{At.mass of copper}}{\text{Eq.mass of copper}}$
$\Rightarrow \large\frac{63.5}{63.5}$$=1$
$\therefore$ Formula of copper chloride=CuCl
Hence (a) is the correct answer.