PIM-SM (Protocol Independent Multicast - Sparse Mode) operates on the principle that multicast traffic is only forwarded to routers that specifically request it. This is achieved through routers with receivers sending join messages to their upstream neighbors, which then build a distribution tree guiding the multicast traffic only to the interested receivers. Therefore, traffic is not initially flooded to all routers, and it is specifically directed to the routers that have requested to join the distribution tree. These descriptions align with options B and D.