Which statement about Auto-RP is true?
A. All interfaces must be configured in sparse mode.
B. All interfaces must be configured in sparse-dense mode.
C. All interfaces must be configured in dense mode.
D. An RP that it tied to a loopback address must be configured.
B
A prerequisite of Auto-RP is that all interfaces must be configured in sparse-dense mode using the ip pim sparse-dense-mode interface configuration command. An interface configured in sparse-dense mode is treated in either sparse mode or dense mode of operation, depending on which mode the multicast group operates. If a multicast group has a known RP, the interface is treated in sparse mode. If a group has no known RP, the interface is treated in dense mode and data will be flooded over this interface.
https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/rps.html#wp1029241
https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/rps.html#wp1029241
B is correct.
Because answer A does not mention listener, I think B is the only answer, at least it works in dense mode.
D is not correct either.
I checked the CCIE v5.0 Volume 1 and several Cisco documents, nowhere does it say that auto-RP has to be tied with an loopback interface, though many examples do use loopback interfaces.
Actually, one can check the command syntax of ip pim send-rp-announce, and you can see that one can use physical interfaces as well.
B is not correct. There is another option
Form page 362 of CCIE v5.0 Volume 1:
To avoid unnecessary dense-mode flooding, configure each router as an
Auto-RP Listener and use sparse-mode on the interface.
A prerequisite of Auto-RP is that all interfaces must be configured in PIM sparse-dense mode
From