Based on the results of Experiment 2, the question that can be answered is: "How is the strength of an electromagnet affected by the voltage of the battery?"
This is because in Experiment 2, the students are using different batteries (thus varying the voltage) while keeping the number of coils constant to see how this affects the electromagnet's ability to pick up paperclips. The other options are either not relevant to the second experiment or reference variables that were not manipulated in that experiment (such as the number of coils or the type of paperclips).