The direction of the flow of multicast traffic is based on which protocol?

A.
PIM

B.
IGMP

C.
underlying IGP

D.
MSDP

Explanation:
https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/mcst_ovr.html
“Reverse Path Forwarding (RPF) PIM uses the unicast routing information to create a distribution tree along the reverse path from the receivers towards the source.
The multicast routers then forward packets along the distribution tree from the source to the receivers. RPF is a key concept in multicast forwarding. It enables
routers to correctly forward multicast traffic down the distribution tree. RPF makes use of the existing unicast routing table to determine the upstream and
downstream neighbors. A router will forward a multicast packet only if it is received on the upstream interface. This RPF check helps to guarantee that the
distribution tree will be loopfree.”