The correct answer is
b. low frequencies and low energy.
In fact, microwaves and radio waves have frequency up to [tex]10^{10}-10^{11} Hz[/tex], which are really low compared, for instance, to the frequency of gamma rays ([tex]10^{20} Hz[/tex]). Moreover, the energy of an electromagnetic wave is directly proportional to its frequency:
[tex]E=hf[/tex]
where h is the Planck constant and f the frequency; therefore, low frequency means also low energy.
