Answer:
If Sam runs are a constant rate, that means his speed won't variate, it'll be the same all the way.
This constant movement is described by a linear relation, which it's shown as a graph line, where the slope of such line describes the constant ratio of change between distance and time, because a constant movement is defined as constance distances covered in constant time intervals.
So, the best description for Sam's movement is a line, which simulates the liner relation between the distance and the time.