#### Answer

$\boxed{\textbf{Is a function.}}$

#### Work Step by Step

A function is a rule that assigns to each element from one set exactly one element from another set.
Written as $\sqrt{x}$, it is understood that we calculate the POSITIVE square root of the number x.
For any non-negative x, there exists one and only one positive square root...
so this rule is a function.
(to each x-value, only one y-value is assigned)