The range consists of the possible y values in a function. In the case of square root functions, they can never be negative. So they y values must be equal to or greater than zero.
(16 - x^2)^.5 >=0
16 - x^2 >=0
x^2 <= 16
x <= +/- 4 or -4 <= x <= 4
For those x values, the y at its lowest is zero & at its highest 4.
Range: 0 <= y <= 4