(d)
Given, $f(x) = x - 3,\ \ g(x) = x^{2} + 1$
$\therefore\ \ \ \ \ \ \ g\left\{ f(x) \right\} = g(x - 3)$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 10 = (x - 3)^{2} + 1$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 10 = x^{2} + 10 - 6x$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ x(x - 6) = 0 \Rightarrow x = 0,\ 6$