If the question asks for the largest set of real numbers that can serve as the domain of the function $f(x)=\sqrt x$, it isn’t really correct to say that $x\ge 0$ is the answer: $x\ge 0$ is a condition on $x$, not a set of real numbers. The answer is $\{x\in\mathbb{R}:x\ge 0\}$ or $[0,\infty)$; both are correct, since they specify exactly the same set. Of course the same information can be expressed in many other ways. To give just one, one can say:
$\sqrt x$ is defined if and only if $x\ge 0$.
If the function were instead $g(x)=\sqrt{x^2-4}$, the answer could be given either in interval notation as $(-\infty,-2]\cup[2,\infty)$ or in set notation as $\{x\in \mathbb{R}:|x|\ge 2\}$; these specify exactly the same set and are equally correct. Yet another possibility is $\{x\in\mathbb{R}:x\le -2\}\cup\{x\in\mathbb{R}:x\ge 2\}$, mirroring the interval notation.