Explanation: The Global Maritime Distress and Safety System (GMDSS) mandates continuous watch on specific Digital Selective Calling (DSC) frequencies to ensure rapid and reliable distress alerting. These frequencies are chosen to cover various ranges and propagation conditions: * **VHF Ch-70**: This channel is dedicated solely for VHF DSC calls, including distress alerts, urgency, safety, and routine calls within line-of-sight range. * **2 MHz MF DSC (2187.5 kHz)**: This is the primary MF (Medium Frequency) DSC distress frequency, used for regional alerting. * **8 MHz HF DSC (8414.5 kHz)**: This is a mandatory primary HF (High Frequency) DSC distress frequency, essential for long-range alerting. * **One other HF DSC frequency**: Vessels must also maintain a watch on one additional HF DSC frequency from the 4, 6, 12, or 16 MHz bands, selected based on geographical area and propagation conditions to optimize long-range communication. Option B correctly lists these mandatory DSC watchkeeping bands/channels, ensuring comprehensive coverage for distress and safety communications. Option A is incorrect because while 6 MHz is an HF DSC frequency, 8 MHz is the specifically mandated primary HF DSC watch frequency. Option A omits 8 MHz. Option C is incorrect because it lists VHF Ch-16, which is a *voice* distress channel, not a *DSC* channel. VHF DSC watch is maintained on Ch-70.