Answer: C. The material
Step-by-step explanation:
In order to check whether a material conducts electricity, the first thing to do would be to design a circuit. Connect a positive wire to the batteries and then another wire from the battery to the bulb and then a wire from the bulb as well.
Then bring different materials to test for conductivity by connecting the wires to the material. If the bulb lights then it is a conductor and if it doesn't the material is not a conductor. The material is therefore the changing variable.