Question

a horizontal line on a distance - time graph represents an object
not moving
decelerating
accelerating
moving at a constant speed

Explanation:

In a distance - time graph, the slope of the line represents speed. The formula for slope \(m=\frac{\Delta y}{\Delta x}\), and in a distance - time graph, \(y\) represents distance (\(d\)) and \(x\) represents time (\(t\)), so \(m = \frac{\Delta d}{\Delta t}=v\) (speed). A horizontal line has a slope \(m = 0\). When \(m = 0\), \(\frac{\Delta d}{\Delta t}=0\), which means \(\Delta d=0\) (no change in distance over time). An object with no change in distance over time is not moving.
For decelerating, the slope of the distance - time graph would be decreasing (becoming less steep in the positive direction). For accelerating, the slope of the distance - time graph would be increasing (becoming steeper). For moving at a constant speed, the slope of the distance - time graph would be a non - zero constant.

Answer:

not moving