40) This question asks us to consider the reaction by the leaves of the electroscope. The problem tells us that the electroscope is uncharged. This doesn't mean that there are no protons or electrons; it means that there are the same amount of protons as electrons.
Let's see what happens when the rod is brought close to the knob of the electroscope.
When a positively charges rod is brought close to the knob, the electrons are attracted to the rod. Since electrons are free to move, they move to the knob because the electrons want to get as close as they can get to the positive rod.
As electrons move towards the knob, the leaves of the electroscope become positively charged and repel each other. The leaves of the electroscope diverge because both are positively charged.
Answer: B
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