Answer
Please see the work below.
Work Step by Step
(a) We know that when the frequency is high, the value of $X_L$ become very large and hence no current flows and it acts like an open circuit. But when the frequency is low, then the value of $X_L$ becomes negligible and the inductor behaves as an ideal wire; the effective resistance of the circuit decreases and thus more current flows through the circuit. Thus, we conclude that the rms current at high frequency is less than the rms current at low frequency.
(b) We know that the best explanation is option (II) -- that is, less current flows at high frequency because then the inductor acts like an open circuit and current has only one path to flow through.
