Final answer:
The output of the function is negative on the entire interval.
Step-by-step explanation:
The function f(x) is a horizontal line and contains the point (8, -6). To determine the interval on which the output of the function is negative, we need to consider the y-coordinate of the point. Since the y-coordinate is -6, which is negative, the output of the function f(x) is negative on the entire interval.
Therefore, the correct option is: C. It is a horizontal line at some negative value.