Explanation: Initiating an ARQ (Automatic Repeat Request) call to a Coast Station and waiting for the automatic exchange of answerbacks is the most comprehensive test for an MF-HF NBDP system. ARQ is a two-way, synchronous digital mode designed for reliable data exchange, where stations confirm reception. Receiving an answerback from a reliable Coast Station verifies that your transmitter, antenna, receiver, and modem are all functioning correctly, and that your signal is successfully being received and decoded by the distant station, confirming full system operability. Option A is incorrect because NBDP is a digital data mode (teletype), not a radiotelephone (voice) system. Option C, initiating an FEC (Forward Error Correction) call, is less effective because FEC is a one-way broadcast mode. While it would demonstrate transmission, you would receive no acknowledgment, so you wouldn't confirm distant reception or your own receiver's functionality. Option B is less complete than D, as the crucial element of "waiting for the automatic exchange of answerbacks" specifically confirms successful two-way communication.