Final answer:
Rubber is an insulator and will not allow electricity to flow easily, so it cannot be used to complete an electrical circuit that requires a conductor.
Step-by-step explanation:
The student's suggestion to use rubber to complete the electrical circuit would not work because rubber is an insulator, not a conductor. Materials that allow electricity to flow through them easily are called conductors, such as metals like copper, silver, and aluminum. In contrast, insulators are materials that do not allow electric charges to move easily through them, with examples including rubber, plastics, glass, and wood.