The correct sentence is:
The taxi is moving with reference to A) Monument circle. For each leg of the trip, the taxi’s A) Average speed stays the same, but its B) Average velocity changes.
In fact, speed it is only the ratio between the distance traveled and the time taken - so it does not take into account the direction of motion. The speed in the first leg is
while the speed in the second leg is
so the speed stays the same.
However, the velocity also takes into account the direction of motion: in the first leg, the direction is east, while in the second leg, the direction is south, therefore the average velocity has changed, since the direction has changed.