Final answer:
In a distance vs. time graph, a line sloping down from upper left to lower right indicates that the object is moving with decreasing speed, heading back towards the starting point.
Step-by-step explanation:
In a distance vs. time graph, the slope of the line represents the velocity of the object. If the line slopes from the upper left to the lower right, this typically indicates that the distance is decreasing over time, hence the object is moving towards the starting point. Therefore, the correct answer is that the object is moving with decreasing speed.
A vertical line would suggest the object is at rest, as the distance does not change over time. A line with a constant positive slope would represent an object moving at a constant speed away from the origin. If the slope were getting steeper over time, it would imply that the object is moving with increasing speed. Since the slope is negative and assuming it's linear, the object is most likely moving with a constant negative velocity, meaning its speed is constant, but since the distance is decreasing, it's heading back towards the starting point.