The correct answer is: A. Impedance
Relays are used for phase faults on long lines to protect the line from damage. When a phase fault occurs, the current in the line increases significantly. This increase in current can be detected by a relay, which will then open the circuit breaker to isolate the faulted section of the line.
Impedance is the opposition to the flow of current. It is a combination of resistance and reactance. Resistance is the opposition to the flow of current due to the resistance of the conductor. Reactance is the opposition to the flow of current due to the inductance and capacitance of the conductor.
When a phase fault occurs, the current in the line increases significantly. This increase in current causes an increase in the impedance of the line. The relay detects this increase in impedance and opens the circuit breaker to isolate the faulted section of the line.
The other options are incorrect because they do not explain how relays work. Option B, reactance, is only one component of impedance. Option C, either of the above, is not a complete answer. Option D, none of the above, is incorrect because relays are used for phase faults on long lines.