Hello.
The answer: D. The conservation of momentum
The conservation of momentum is for a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the momentum gained by object 2.
Have a nice day
It could be, depending on WHAT distance was graphed,
and how the student labeled the graph's axes.
Most likely, however, she probably drew the graph ...
-- with time on the x-axis increasing to the right,
-- with distance on the y-axis increasing upward,
-- to show the distance FROM the starting point
NOT the distance remaining to the destination.
If that's how she set it up, or if any two of these items are
the other way around, then the line could not slope downward.
THAT would say that the distance from the starting point is
decreasing as time goes on, or the distance remaining to the
destination is increasing as time goes on. That would be silly.