Answer:
B. Mary stopped and waited for the traffic light to turn green.
Explanation:
Lets check the other options to see if distance changes:
A. Mary used her bike to ride the first mile. Definitely distance changes with time.
C. Mary walked her bike across the avenue and up a short hill. Here a short distance is covered with respect to time.
D. Mary rode her bike at a constant speed for one mile. As she is riding, so definitely the distance is changing.
Therefore, only option B is the point where there is no movement and no change in distance.