Ans: (D)
Hint : \(f(x)=\sqrt{(x-1)(x-2)}, g(x)=\sqrt{x}\)
$$
\begin{aligned}
&f(g(x))=\sqrt{(\sqrt{x}-1)(\sqrt{x}-2)} \\
&S=\{x: x \in[0,1] \cup[4, \infty)\} \\
&g(f(x))=\sqrt{\sqrt{(x-1)(x-2)}} \\
&T=\{x: x \in(-\infty, 1] \cup[2, \infty)\}
\end{aligned}
$$