Concept:The electromotive force (emf) is the energy supplied by a source to drive one unit of charge once around the whole circuit.
Explanation:The question asks for the energy needed to move a unit positive charge around a complete electric circuit.
This is exactly the definition of electromotive force, or emf.
A source such as a battery or generator does work on each unit of charge to push it around the entire closed loop.
Electric potential difference, however, is the work per unit charge between two specific points in the circuit, not around the whole circuit.
Electric energy refers to the total energy transferred, not the energy per unit charge.
Kinetic energy is the energy an object has because of its motion, which is unrelated to moving charge around a circuit.
Therefore, the correct option is the one that matches the definition of emf.
Answer:A. electromotive force