Answer:
B. a ball on a string that stopped at the top of a vertical loop
Step-by-step explanation:
Motion diagram must represent the direction of velocity of an object in the given figure
here as it is shown in the figure the arrow is vertically downwards which means that the velocity of object at that moment is downwards when it is at the top most point of its circular path
So here
A. a car driving over the top of a hill
Not correct as car is moving tangentially on the top of the hill so its velocity can not be downwards
B. a ball on a string that stopped at the top of a vertical loop
Since ball is stopped at the top most point of its path so here its velocity must be downwards due to free fall so its correct motion diagram for the above condition.
C. a roller coaster car at the top of a vertical loop of a track
Not correct as we know that roller coaster at the top of the loop will move tangentially so its velocity can not be vertically downwards.
D. a bug on the top of a rolling hula hoop
Bug is at the top of rolling hula hoop so its velocity is again tangentially and not vertical downwards.