(d)
Let $z = a + ib$
$\therefore\ \ \arg{(z) = \theta = \tan^{- 1}\frac{b}{a}}$
$\because\ \ \ \overline{z} = a - ib$
$\therefore\ \arg{\left( \overline{z} \right) = \tan^{- 1}{\left( - \frac{b}{a} \right) = - \tan^{- 1}{\left( \frac{b}{a} \right) = - \theta}}}$