The lines on the position-time graph show the velocities of different vehicles. which line represents a vehicle moving at constant velocity?

Any straight line, like the orange one or the green one, shows constant speed. The orange line shows a constant speed backwards ... as time goes on, the position or distance from something gets smaller. The green line may show a constant speed of zero ... as time goes on, the position doesn't change.

Really, none of these graphs can show a velocity, only speed. To know velocity, you need the direction of the motion, and that's not on the graph.

Did you ever figue this