In MODEMS: 

The correct answer is a digital signal changes some characteristic of a carrier wave

Key Points

• A modem's primary role is to convert digital signals to analog for transmission over an analog medium (modulation), and to convert received analog signals back into digital form (demodulation).
• Modems impress digital data on a carrier wave by altering certain characteristics of that wave - which could be its amplitude, frequency, or phase.
• While some modems may also amplify signals or multiplex multiple data streams, these are not core functions of a modem and typically require supplementary hardware or advanced modem designs.
• Hence, they do not accurately represent the primary function of a modem.
• The correct answer would be: A digital signal changes some characteristic of a carrier wave