Which two factors should the students vary to best investigate the strength of the force between the paper clip and a magnet

Many young students will have had memorable but often confusing experiences with magnets and magnetic materials. Magnetic materials are regularly encountered around the home, often holding small objects on the kitchen fridge or keeping cupboards and fridge doors closed. Many children’s toys employ weak magnets to ‘stick’ materials together (e.g. wooden train carriages) or are used in simple children’s construction toys to allow them to quickly assemble more complex structures without the need for messy adhesives or difficult connections. Toys very rarely make use of magnetic repulsion.

Explanation:

it is the second option.

d.7.5cm is your answer