A zero slope because the gradient is zero
That slope is a B. Zero Slope. You know this because you measure slopes through the gradient of a line. The formula for this is: Rise/Run Rise is how far it moves up the y axis, and run is how far it goes through the x axis. The rise of a straight line is 0, and the run of the straight line is infinite. So, you can say that because anything divided by zero is zero, the slope of a horizontal straight line is 0!