______helps to maintain potential difference across a conductor.

Correct option is C

The correct answer is (c) Battery.
Explanation: 

A battery provides the necessary electromotive force (EMF) that helps to maintain a potential difference (voltage) across a conductor. It drives the current through the circuit by creating this potential difference between its two terminals.

• A voltmeter measures the potential difference, but it does not create or maintain it.
  
•  A plug key is used to open or close the circuit, but it doesn't maintain potential difference.
  
• Resistance affects the flow of current in the circuit, but it does not create a potential difference.

Thus, the battery helps to maintain the potential difference across a conductor.
Conclusion: The correct answer is (c) Battery.