Compute $f\left(16\right)$ for $f\left(x\right)=\sqrt{x}$ and explain why this input is allowed.

$f\left(16\right)=4$, because $16$ is a nonnegative real number and is in the domain of the square root function

$f\left(16\right)=-4$, because both positive and negative numbers square to $16$

$f\left(16\right)=\pm 4$, because square roots always produce two values

$f\left(16\right)$ is undefined, because $16$ is not in the domain of the square root function