Which action enables an IPv6 multicast router on a subnet with locally attached receivers to quickly determine if there is a need to send traffic for a certain multicast group out to that subnet?

Adjusting the MLD report interval affects how frequently receivers send reports but does not provide the immediate pruning capability of fast-leave when a receiver leaves a group.

MLD snooping (the IPv6 equivalent of IGMP snooping) combined with fast-leave functionality enables a Layer 2 device to immediately remove a port from a multicast group when a leave message is detected, preventing unnecessary traffic forwarding. This mechanism allows the router to quickly adapt to receiver changes on the subnet and stop sending traffic for groups that no longer have local listeners, effectively determining the need for traffic.

MLDv2 explicit-tracking enhances source-specific multicast (SSM) by allowing the router to track specific (S,G) interests but does not primarily address the quick cessation of group traffic when receivers leave a group in the general sense.

ICMPv4 and its query timer are relevant to IPv4 multicast, not IPv6, making this option inapplicable to an IPv6 multicast router.