Final answer:
The slope of the line determined by the equation y = 8 is zero, indicating it is a horizontal line at a positive value on the Cartesian plane.
Step-by-step explanation:
The slope of the line identified by y = 8 is zero.
This equation represents a horizontal line on the Cartesian plane since it does not change in vertical position; for every value of x, y remains constant at 8.
There is no rise over run, indicating that the slope, which represents the steepness of the line, is zero (0).
Thus, y = 8 is a horizontal line at some positive value, and the correct answer is D. 0.