Final answer:
The ICMPv6 message sent when the IPv6 hop limit field of a packet is decremented to zero and the packet cannot be forwarded is the Time Exceeded message.
Step-by-step explanation:
The ICMPv6 message that is sent when the IPv6 hop limit field of a packet is decremented to zero and the packet cannot be forwarded is the Time Exceeded message. This message is generated by a router when it receives an IPv6 packet with a hop limit of zero.
The Time Exceeded message is used to inform the source host that the packet was dropped due to the hop limit being exceeded. It helps to prevent packets from looping indefinitely in the network.
When the Time Exceeded message is received, the source host can use the information in the message to determine the path the packet took and identify potential routing issues.