Explanation: The principle advantage of a higher AC line frequency, like 400 Hz used in aircraft, relates directly to the size and weight of magnetic components. Inductive reactance ($X_L = 2\pi f L$) is directly proportional to frequency ($f$). For a given required inductance or impedance, a higher frequency allows for fewer turns of wire or a smaller magnetic core in components like transformers, chokes, and filters. This reduction in physical size and weight is crucial for aircraft and avionics, where every ounce and cubic inch saved contributes to better fuel efficiency, payload capacity, and overall performance. Therefore, answer (C) correctly identifies that magnetic devices are smaller and lighter. Answers (A) and (B) are generally incorrect; the current drawn depends on the power delivered, and efficiency isn't inherently better just due to higher line frequency (core losses can even increase). Answer (D) is incorrect; 400 Hz systems are specialized and typically more expensive to produce than standard 60 Hz equipment.