Final answer:
In a position vs. time graph, uniform motion is represented by a straight non-horizontal line, indicating constant velocity; a flat horizontal line represents a stationary position.
Step-by-step explanation:
In a position vs. time graph, different relationships represent various types of motion. When an object is depicted with a straight and non-horizontal line on such a graph, it indicates uniform motion, meaning the object's rate of change of position with time remains constant and its position changes by the same amount in equal time intervals. This represents constant velocity, where (a) is true. A flat horizontal line would represent a stationary position, where the object is at rest or its position doesn't change over time. Whereas, if the slope of the line is changing, the graph suggests acceleration since the rate of change of velocity with time remains constant. This can be deduced from both the position vs. time graph and the velocity vs. time graph, whose slope signifies acceleration.