Final answer:
A device obeying Ohm's law must have a constant resistance, as the resistance acts as the constant of proportionality between the current (I) and the voltage (V) applied across the device.
Step-by-step explanation:
Ohm's law states that the current (I) through a conductor between two points is directly proportional to the voltage (V) across the two points, with the resistance (R) being the proportionality constant. Therefore, a device obeying Ohm's law must have a constant resistance, which is option B. The formula representing Ohm's law is V = I × R, implying that as the voltage increases, the current increases if the resistance remains constant. Likewise, if you increase the resistance while keeping the voltage constant, the current will decrease. Thus, resistance is not directly proportional to either current or voltage but acts as the factor that determines how much current flows for a given voltage.