Final answer:
The letter E stands for electromotive force (emf). It is the voltage or electric potential difference in a circuit that drives current through the circuit.
Step-by-step explanation:
The letter E in the context of this question stands for electromotive force (emf). The term electromotive force refers to the voltage or the electric potential difference in a circuit that is needed to drive current through that circuit. It is measured in volts (V).
For example, in the question provided, option a. electromotive force is the correct answer. Electromotive force is associated with the voltage provided by a car battery or any other source of electrical energy.