Internet Group Management Protocol Version 2 (IGMPv2) introduced the explicit "Leave Group" message. This feature allows a multicast client to actively send a request to the local multicast router, signaling its intent to stop receiving traffic for a particular multicast group. Upon receiving this message, the router can initiate a query to see if any other members remain on the subnet. This makes the process of leaving a group significantly faster and more efficient compared to IGMPv1, which relies on a passive timeout mechanism where the router must wait to stop forwarding traffic.

B. IGMPv1: This version lacks an explicit "Leave Group" message. A host simply stops responding to membership queries, and the router only stops forwarding traffic after a timeout period expires.

C. PIM Dense: This is a multicast routing protocol that operates between routers to build distribution trees. It is not used by end-clients on the local subnet to manage group membership.

D. IGMP Snooping: This is a Layer 2 switch feature that optimizes multicast traffic delivery by listening to IGMP messages. It is not the protocol used by the client to send the leave request.

1. RUCKUS FastIron Layer 3 Routing Configuration Guide, 08.0.95, Chapter 25: Configuring IP Multicast, section "IGMPv2", page 628. The document states, "IGMPv2 adds a Leave Group message to the protocol. When a host wants to leave a multicast group, it sends a Leave Group message to the all-routers multicast group (224.0.0.2)."

2. IETF RFC 2236, "Internet Group Management Protocol, Version 2", November 1997, Section 1: Introduction. This RFC, which defines the protocol, explicitly states: "Version 2 of IGMP adds a 'leave group' message to allow a host to inform a router that it wishes to leave a multicast group..."

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In Chapter 4, Section 4.7.2, the text explains the IGMP protocol, noting that IGMPv2 (and v3) added messages that allow a host to explicitly signal it is leaving a group, an improvement over IGMPv1's timeout-based approach.