Explanation: SITOR (NBDP) signals are generated using Frequency Shift Keying (FSK), where the carrier frequency is shifted between two distinct frequencies. The standard shift for SITOR is 170 Hz, meaning the carrier shifts +85 Hz for one state (e.g., mark) and -85 Hz for the other state (e.g., space) relative to a center frequency. Shipboard GMDSS equipment often uses general-purpose HF SSB (Single Sideband) transceivers. To generate an FSK signal with an SSB transmitter, Audio Frequency Shift Keying (AFSK) is employed. This involves modulating the SSB transmitter with two distinct audio tones, which are then translated into the corresponding RF frequencies. * **Option A** describes this method: "Two alternating sidebands of 1615 Hz and 1785 Hz are offset from the carrier frequency by 1700 Hz." * The two audio tones, 1615 Hz and 1785 Hz, have a frequency difference of 170 Hz (1785 - 1615 = 170 Hz). This matches the standard SITOR shift. * When an SSB transmitter (typically in USB mode) is modulated with these tones, the tones themselves become the transmitted RF frequencies relative to the SSB suppressed carrier. * The "offset from the carrier frequency by 1700 Hz" refers to the nominal SSB suppressed carrier frequency. If an SSB radio considers a 1700 Hz audio tone to be at its "dial frequency," then 1615 Hz will be $1700 - 1615 = 85$ Hz below the dial frequency, and 1785 Hz will be $1785 - 1700 = 85$ Hz above the dial frequency. This produces the required $\pm 85$ Hz shift. This is the common method for SITOR generation via SSB radios. * **Option B** ("The carrier frequency is alternately shifted from + 85 Hz to – 85 Hz while suppressing the sidebands.") describes the *effect* of FSK (the direct shift of the carrier) but not the specific *method of generation* commonly used in multi-mode GMDSS equipment via SSB. While the shift values are correct for SITOR, the statement regarding "suppressing the sidebands" is somewhat ambiguous in this context. FSK inherently creates sidebands. Option A is a more precise description of the generation method using SSB. * **Option C** ("There is one 1500 Hz sideband which is shifted from +170 Hz to – 170 Hz with a suppressed carrier.") is incorrect. A total shift of 340 Hz (170 Hz to -170 Hz) is not the standard 170 Hz SITOR shift. * **Option D** ("Two alternating sidebands of 1500 Hz and 2300 Hz are offset from the carrier frequency by 1900 Hz.") is incorrect. The difference between 1500 Hz and 2300 Hz is 800 Hz, which is not the standard 170 Hz shift for SITOR/NBDP. Therefore, Option A accurately describes the standard AFSK-over-SSB method for generating SITOR signals in GMDSS equipment.