Answer
The student can draw the conclusion that the charge of the electron should be the difference between the charges of the droplets.
Work Step by Step
Since the difference of the charges between A&B, B&C and C&D are all relatively similar ($1.55*10^{-19}$, $1.66*10^{-19}$ and $1.50*10^{-19}$), the student can infer that each has a difference of the charge of one electron.
