An efficient and a well designed protective relaying should have

The correct answer is D. all of the above.

An efficient and a well designed protective relaying should have good selectivity and reliability, economy and simplicity, high speed and selectivity.

• Selectivity: The ability of a relay to distinguish between a fault on its protected circuit and faults on other circuits.
• Reliability: The ability of a relay to operate correctly when a fault occurs.
• Economy: The cost of the relay should be reasonable in relation to its benefits.
• Simplicity: The relay should be easy to understand and operate.
• High speed: The relay should operate quickly enough to clear a fault before it causes extensive damage.

All of these factors are important in the design of an efficient and well-designed protective relaying system.