Correct option is (a) \(73I_{rms}^2\)
To explain I would say: Radiation resistance of a half-wave dipole is 73Ω.
Average Power radiated from the half-wave dipole \(P_{avg}=I_{rms}^2 R=73I_{rms}^2\)
Radiation resistance of a quarter-wave monopole is 36.5Ω.