Answer: C) Wrap more turns of wire around the nail.
Step-by-step explanation:
One of the main determinants of the strength of a magnet is the number of rounds of wire around the magnet. This is because, the more wire there is, the more electricity will pass through and the more powerful the magnet will become.
They should therefore wrap more turns of wire around the nail so that more electricity can flow from the battery. This will make their magnet stronger and enable it to pick up more paper clips.