The correct answer is: A. Impedance relay.
An impedance relay is a type of protective relay that is used to protect transmission lines from faults. It works by comparing the impedance of the line to a set value. If the impedance of the line deviates from the set value by too much, the relay will trip the circuit breaker, isolating the faulted section of the line.
Impedance relays are typically used on long transmission lines because they are more sensitive to faults than other types of relays. They are also less likely to be affected by transients, which can cause other types of relays to trip incorrectly.
Mho’s relay and reactance relay are also types of protective relays, but they are not as commonly used on long transmission lines as impedance relays. Mho’s relays are used to protect against faults that cause a change in the current flow through the line, while reactance relays are used to protect against faults that cause a change in the voltage across the line.
None of the above is not a correct answer.