The correct answer is: B. sensitive
A sensitive relay is a relay that is able to detect small changes in current or voltage. This is important for discrimination between main and backup protection, as the main protection system needs to be able to quickly detect a fault and trip the circuit breaker, while the backup protection system needs to be able to detect a fault that the main protection system may have missed.
A fact relay is a relay that is used to detect faults in a power system. It is typically used in conjunction with a circuit breaker to trip the breaker if a fault is detected. Fact relays are not typically used for discrimination between main and backup protection.
A slow relay is a relay that takes a longer time to operate than a fast relay. This is because a slow relay is designed to be more tolerant of transients and other disturbances on the power system. Slow relays are not typically used for discrimination between main and backup protection.
Therefore, the correct answer is: B. sensitive