Which represents the reflection of f(x) = square root of x over the x-axis?

"sqrt" is shorthand for "square root"
So writing "sqrt(x)" means "square root of x"

The original function is f(x) = sqrt(x)

The answer is g(x) = -sqrt(x)

All we do is stick a negative in front to flip all the y coordinates from positive to negative. For instance, the point (9,3) is on f(x) while (9,-3) is on g(x). All that changes is the y coordinate from positive to negative. The x coordinate stays he same.