Explanation: For transmissions made in radiotelegraphy (Morse code), particularly on the international distress frequency of 500 kHz, international radio regulations mandate that a specific auto-alarm signal precede the distress call. This signal, consisting of two alternating tones (2200 Hz and 1300 Hz for 250 milliseconds each, sent for 30-60 seconds), is designed to activate automatic alarm devices on ships and coast stations and alert watchkeeping operators, ensuring that the subsequent distress call (SOS) receives immediate attention. Option B is incorrect because "SOS" sent three times *is* the distress call itself, not a preceding signal. Option C is incorrect as "MAYDAY" is the distress call used in radiotelephony (voice communications), not radiotelegraphy. Therefore, the auto-alarm signal is the specific transmission that precedes a radiotelegraph distress call.