(d)
Let $z = x + iy$
$\therefore\ \ \ \ |z + 3 - i| = \left| (x + 3) + i(y - 1) \right| = 1$
$\Rightarrow \ \ \ \ \ \sqrt{(x + 3)^{2} + (y - 1)^{2}} = 1$ …(i)
$\because\ \ \ \arg{z = \pi\ \ \ \ \ \ \ \ \ \ \ \Rightarrow \ \ \ \ \tan^{- 1}{\frac{y}{x} = \pi}}$
$\Rightarrow \ \ \ \ \ \ \frac{y}{x} = \tan\pi = 0\ \ \ \ \$ $\Rightarrow \ \ y = 0\ \$ …(ii)
from Eqs.(i)and (ii)we get
$x = - 3,\ \ \ y = 0$
$\therefore\ \ \ z = - 3$
$\Rightarrow \ \ \ |z| = | - 3| = 3$