Which form of equation represents a horizontal line, and why does that form correspond to a slope of zero?

xy = c, where the product is constant; the slope is 0 because the product constraint makes vertical changes impossible.

y = mx + b, where m is undefined; the slope is 0 when y is constant for all x because the m term cancels out.

x = c, where c is a constant; the slope is 0 because the x-value never changes, which means the line is perfectly flat and horizontal.

y = c, where c is a constant. The slope is 0 because Δy = 0 for all points, and 0 divided by any nonzero Δx equals 0.