Final answer:
A black band on a resistor represents the digit 0 in the resistor color-coding system used to indicate resistance values. The correct answers in option a.
Step-by-step explanation:
The digit represented by a black band on a resistor is 0. In the color-coding system used for resistors, colors represent numbers where black corresponds to 0, brown to 1, red to 2, and so on. Therefore, the answer is option 4) 0. This color code is a standardized way to indicate the resistance values of resistors. For example, a resistor with bands of brown, black, and red would have a resistance of 10Ω with a multiplier of 100, making it a 1 kΩ resistor.