Explanation: DSC (Digital Selective Calling) circuits transmit information as digital data. Digital data is represented by changes in voltage levels over time, corresponding to the "high" and "low" states of the binary bits (0s and 1s). When an outgoing message is being transmitted, there is a continuous stream of these bits. A voltmeter connected to such a data line would register these rapid voltage changes as a "fluctuating" reading. The 0 to 2 volts DC range is typical for the low-voltage logic levels found in marine digital data interfaces, such as NMEA 0183, which commonly connect to DSC radios. A steady voltage (A) or an absence of voltage (C) would indicate an idle line or no data transmission. A voltage swing from -12 VDC to +12 VDC (D) is characteristic of different, older serial standards (like RS-232) and is not typical for the data interfaces used in modern marine DSC equipment.