The correct option is Cell.
Explanation
For a continuous flow of electric current through a conductor, a constant potential difference must be maintained across its ends. This function is performed by a source of electromotive force (EMF).
- Cell: A cell (or battery) converts chemical energy into electrical energy. Inside the cell, chemical reactions generate a potential difference between the two terminals (positive and negative). This potential difference acts as the driving force that moves charge carriers (electrons) through the circuit, thereby maintaining the current.
- Ammeter: An instrument used strictly to measure the magnitude of electric current flowing in a circuit. It does not generate or maintain potential difference.
- Resistor: A passive component that opposes the flow of current. It dissipates electrical energy in the form of heat or light but does not generate the potential difference required to drive the current.
- Switch: A device used to make or break the electrical connection in a circuit. It controls the flow of current but is not the source of the potential difference.
Key Takeaway: A cell or battery acts as the electrical pump in a circuit, providing the necessary energy to maintain a potential difference across the terminals, which enables the continuous flow of charge.