(a)
Eq. mass of copper chloride = 99
Eq. mass of chlorine = 35.5
$\therefore$ Eq. mass of copper = 99 $- 35.5 = 63.5$
$\therefore$ Valency of copper $= \frac{at\ mass\ of\ copper}{eq.mass\ of\ copper} = 1$
$\therefore$ Formula of copper chloride is $CuCl.$