Explanation: Before making a transmission, a station operator must determine that the frequency is not in use by monitoring it. This is a fundamental principle of good amateur radio operating practice, rooted in FCC Part 97 regulations which emphasize avoiding interference. Listening first ensures you don't disrupt ongoing communications or transmit over another station, promoting efficient and polite use of the shared radio spectrum. The only exception is for distress calls, which take immediate priority. Transmitting a general notification (B) or asking if the frequency is in use (D) without first monitoring could still cause interference. Checking equipment calibration (C) is important for signal quality but does not address frequency occupancy.