Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
According to Sharon's frame of reference, there are two forces exerted on the charge. The magnetic force that acts downward and the electric force that acts upward.
These two forces are equal in magnitude and opposite in direction, so the net force exerted on the charge is zero.
$$\color{blue}{\bf [a]}$$
According to Bill's frame of reference, the charge is at rest which means that there are no forces exerted on this charge.
