I think it would be d because the spring or whatever was pushing until it reached the farthest it could then it would pull down but idrk
Let vb be the velocity of the motorboat and let vs be the velocity of the stream.
We know that when she drives upstream the velocity is 8 m/s, in this scenario the velocities point in opposite directions, then we have the equations:

When she drives downstream the velocites point in the same direction then we have the equation:

hence we have the system of equations:

Solving the first equation for the velocity of the boat we have:

Plugging this in the second equation we have:

Therefore, the velocity of the stream is 2 m/s
So the heavy skateboarder will move off with speed 2.45 m/s in the opposite direction
None whatsoever.
You're confusing the formation of the Earth with the so-called "Big Bang".
The sun and planets formed billions of years AFTER the Big Bang.