Answer:
C. Andreas can wrap many coils of the wire, close together, around the nail.
Step-by-step explanation:
The ideal in this case is for Andreas to wrap the entire nail with the wire. For that he will need to use many coils of the thread. These coils should be close together, touching each other, but without overlapping, they should be next to each other. That way, Andreas's experiment will work.
If he wants to increase the magnetism power of his electromagnet, he must replace the battery with a higher voltage one.