Answer
The charge transfer through the wire will be $-3e$ and this charge will flow upward.
Work Step by Step
Let $q$ be the initial charge of each bubble.
After bubble A hits the ceiling, the charge on bubble A is zero.
After bubble A hits bubble B, both bubbles have the same charge.
The charge on bubble A and bubble B is $\frac{q}{2}$
After bubble B hits bubble C, both bubbles have the same charge.
The charge on bubble B and bubble C is $\frac{\frac{q}{2}+q}{2}$
In part (a), we found that the initial charge of each bubble was $~~+4e$
We can find the charge on bubble B when it hits the floor:
$\frac{\frac{q}{2}+q}{2} = \frac{\frac{+4e}{2}++4e}{2} = +3e$
Therefore, the charge transfer through the wire will be $-3e$ and this charge will flow upward.
