Ball A has a negative charge. Ball B is neutral. Ball C has a positive charge. Ball D is neutral.
Work Step by Step
When a plastic rod is rubbed with wool, electrons move from the wool to the rod, so the rod becomes negatively charged. When the rod touches ball A, then ball A becomes negatively charged. Since balls B, C, and D are attracted to ball A, they could be positively charged or neutral. Since balls B and D have no effect on each other, they must both be neutral. Since ball B (which is neutral) is attracted to ball C, then ball C must be positively charged.