Which protocol uses shared distribution trees and is described in RFC 7761?

The correct answer is PIM-SM, which stands for Protocol Independent Multicast - Sparse Mode. This protocol utilizes shared distribution trees, also known as multicast trees, to efficiently route multicast traffic across a network. The shared distribution tree allows multiple sources to send traffic to a common multicast group, which simplifies routing within sparse networks where multicast group members may not be densely located.

RFC 7761 defines operational aspects of PIM-SM, including recommendations for its implementation and enhancement of multicast capabilities in IP networks. By using a shared distribution tree, PIM-SM allows for optimal routing and minimizes bandwidth consumption by ensuring that multicast streams are only sent to parts of the network where there are active receivers subscribed to that multicast group.

Other options, while related to network protocols, do not utilize shared distribution trees in the same manner. PIM-DM, for instance, operates on a different principle suited for dense environments and does not focus on shared trees. IGMP is a protocol for managing membership in multicast groups and does not handle traffic routing, while RTP is used for delivering audio and video over IP networks but does not relate directly to the distribution tree concept.