Answer
The total charge transfer through the wire is $-12e$ and this charge flows upward.
Work Step by Step
In part (a), we found that the initial charge of each bubble is $+4e$
After bubble A hits the ceiling, the charge on bubble A is zero.
The charge transfer through the wire is $-4e$ and this charge flows upward.
When bubble C hits the floor, the charge transfer through the wire is $-3e$ and this charge flows upward.
When bubble B hits the floor, the charge transfer through the wire is $-3e$ and this charge flows upward.
When bubble A hit the floor, the charge transfer through the wire is $-2e$ and this charge flows upward.
The total charge transfer through the wire is $-12e$ and this charge flows upward.
