The location and the slope of a line on a velocity-time graph is an indication of how an object is moving. A horizontal line (zero slope) indicates a constant velocity. The further such a line is above or below the axis, the faster that the object is moving. A line located above the axis of a v-t graph represents an object with a positive velocity and a line below the axis represents an object with a negative velocity. A sloped line is an indicator of a changing velocity. If the line slopes towards the axis, then the object is slowing down. And if the line slopes away from the axis, then the object is speeding up.
 

You have to match the motions of A, B, and C to one of the six lines on the position-time graph. That's going to require some good thinking and decision-making. The following should help:

Positive vs. Negative Velocity: For each object, you need to decide whether the velocity is positive or negative. Look at the arrow on the dot diagram; it indicates the direction that the object is moving. Typically a leftward motion is considered a negative velocity and a rightward motion is a positive velocity. On a v-t graph, a positive velocity is represented by a line located above the time axis in the positive velocity region (lines 1, 2, or 3). A negative velocity is represented by a line located below the time axis in the negative velocity region (lines 4, 5, or 6).

Fast vs. Slow: A constant speed object can be described as being fast or slow. On a dot diagram, fast corresponds to a large distance between adjacent dots and slow corresponds to a small distance between adjacent dots. You will have to match the fast object to the line on the velocity-time graph that is furthest from the time axis. The slower object must be matched to a line that is closest to the time axis.