Final answer:
PIM-DM is designed for environments with a high concentration of multicast group members and is used when there is a high ratio of routers with attached members relative to the total number of routers.
Step-by-step explanation:
PIM-DM (Protocol Independent Multicast - Dense Mode) is used when the number of routers with attached members is high relative to the number of routers in the internet. In this context, 'attached members' refers to hosts (end systems) that are interested in receiving multicast traffic. PIM-DM is optimized for scenarios where multicast group members are densely packed, meaning there is a high concentration of receivers. This is in contrast with PIM-SM (Protocol Independent Multicast - Sparse Mode), which is designed for environments where group members are sparsely located, and thus the number of routers with attached members is low.