Final answer:
The circuit element with an impedance of z = -4Ω is a capacitor, as the negative sign suggests capacitive reactance in AC circuits. Thus, the correct answer is C. A capacitor.
Step-by-step explanation:
The impedance of an element specified as z = -4Ω (ohms) suggests that this element is reactive rather than resistive, as the negative sign is typically associated with capacitive reactance in AC (alternating current) circuit analysis. Therefore, the correct answer is that the element with an impedance of z = -4Ω is C. A capacitor.
To further clarify, impedance is a measure of how much a circuit element resists the flow of alternating current and can combine both resistance and reactance. Resistors have only resistance and their impedance is real and positive, whereas inductors and capacitors have reactance. The impedance of a capacitor is negative imaginary in phasor notation and decreases with increasing frequency. On the other hand, the impedance of an inductor is positive imaginary and increases with increasing frequency. Since the given impedance is negative, the element is not an inductor, which would have a positive impedance, or a resistor, which would have a real impedance without an imaginary part.