If f(x) = √(x) and g(x) = x - 5, what is the domain of f(g(x))?

Form the composition: $f(g(x))=\sqrt{g(x)}=\sqrt{x-5}$. The expression under the radical must be non-negative: $x-5\ge0$, so $x\ge5$, choice B. Check: at $x=5$ the value is $\sqrt0=0$, and at $x=4$ it would be $\sqrt{-1}$, which is not real. Choice A ($x\ge0$) applies the radical restriction to $x$ instead of to $g(x)$, forgetting the shift by $5$. Choice C ($x\le5$) reverses the inequality. Choice D (all real numbers) is the domain of $g$ alone, ignoring the square root.