Explanation: Back EMF, or back electromotive force, is a voltage generated within an inductive component, such as an inductor or the windings of a motor, whenever the current flowing through it changes. According to Lenz's Law, this induced voltage always acts in a direction that opposes the change in current that caused it. Consequently, it directly opposes the applied voltage (EMF) driving that current change. This is a fundamental concept in AC circuit theory and motor operation. Therefore, option C is correct: back EMF is a voltage that opposes the applied EMF. Options A and D are incorrect because back EMF is a *voltage* (an electromotive force), not a current. Option B incorrectly ties back EMF to an RC time constant, which is relevant to resistor-capacitor circuits, not the inductive effect of back EMF.