Radiation resistance divided by transmission resistance
Radiation resistance divided by total resistance✓Correct answer
Total resistance divided by radiation resistance
Effective radiated power divided by transmitter output
Explanation
Power fed to an antenna divides between radiation resistance, which represents energy that leaves as radio waves, and loss resistance in conductors, insulators, matching components and the ground. Efficiency is the fraction that goes to radiation, so it is radiation resistance divided by the total of radiation plus loss resistance.