The correct answer is a) electric force.
Electromotive force (EMF) refers to the voltage generated by a source like a battery or a generator. It’s not just about the potential difference; it specifically indicates the energy provided per charge by the source, acting like an electric force that pushes the charges through a circuit. While options like charge flow rate and energy measure are related concepts, they don’t encapsulate what EMF fundamentally represents. Thus, identifying EMF with electric force provides a clearer understanding of its role in electrical circuits.