Explanation: MF-HF Digital Selective Calling (DSC) utilizes a specific type of Frequency Shift Keying (FSK) modulation. The international standard for DSC, which is part of the Global Maritime Distress and Safety System (GMDSS), specifies a center (sub-carrier) frequency of 1700 Hz. The data is transmitted by shifting the carrier frequency up or down from this center frequency. For MF-HF DSC, the frequency shift is +/- 85 Hz. This means the "mark" frequency is 1700 Hz + 85 Hz = 1785 Hz, and the "space" frequency is 1700 Hz - 85 Hz = 1615 Hz. This particular shift and sub-carrier are standardized for reliable communication in maritime distress and safety applications. The other options present combinations of frequency shifts and sub-carriers that are not correct for MF-HF DSC. For example, +/- 170 Hz is a common shift for radioteletype (RTTY) but not for DSC.