Explanation

(d)
Given, $f(x) = \frac{1}{\sqrt{- x}}$
$\therefore\ \ \ fof(x) = f\left( f(x) \right) = f\left( \frac{1}{\sqrt{- x}} \right)$
$\Rightarrow \ \ \ \ fof(x) = \frac{1}{\sqrt{- \frac{1}{\sqrt{- x}}}}$
Since, $\sqrt{- \frac{1}{\sqrt{- x}}}$ is an imaginary.
Hence, no domain of $fof(x)$ exist.
Thus, the domain of $fof(x)$ is an empty set.