Concept:GSM is a digital mobile network, and its key distinguishing advantage is that users can communicate while on the move.Explanation:GSM stands for Global System for Mobile Communication.Unlike fixed telephone lines, GSM allows users to move freely from one location to another without losing connection.This portability and ease of carrying the device make mobility its most distinctive advantage.Cost, stability, and coverage are not as unique to GSM because other communication systems can also offer them.Answer:D. Mobility