B is correct : From ACSP learning book Multicast Routing : "PIM-SM builds the multicast routing tree for a group in advance and then forwards multicast on that tree when a source begins to stream to that group. PIM-SM enabled routing interfaces must specifically indicate that they want to receive multicast traffic in a particular group by sending a JOIN for the three" A is incorrect because "pruning" is a PIM-DM concept C is incorrect because PIM-SM : From ACSP learning book Multicast Routing : "Because PIM-SM establishes efficient forwarding paths, it is well suited for a lower bandwidth environment, such as one built on WAN links. D is incorrect: : From ACSP learning book Multicast Routing : "The best way of comparing the two protocoles is PUSH (PIM-DM) versus PULL (PIM-SM)"

https://www.arubanetworks.com/techdocs/AOS-CX/10.07/HTML/5200-7876/Content/Chp_pim-sm/how-pim-sm-wor-10.htm: In a PIM domain, each PIM interface on a router periodically multicasts PIM hello messages to all other PIM routers (identified by the address 224.0.0.13 for V4 and ff02::d for V6) on the local subnet. Through the exchanging of hello messages, all PIM routers on the subnet determine their PIM neighbors, maintain PIM neighboring relationship with other routers, and build and maintain shortest path trees (SPTs).

Correct Answer: B https://www.youtube.com/watch?v=PhzMcUcS6UA