Answer: d. the radius of the Ferris wheel
Explanation:
We now that the amplitude is the vertical distance between a peak or a valley and the equilibrium point.
Thus in the case of Ferris wheel the amplitude should be the difference between the peak(highest) and equilibrium point or equillibrium point and valley(lowest) points .
i.e. it should be half of the diameter of the circle. Since half of diameter of a circle is equals to radius.
Therefore, the amplitude of the function you use as a model should be equal to the radius of the Ferris wheel.