Answer: Router's prefix
Explanation: DHCPv6 (Dynamic host configuration protocol version 6) is the protocol used for an IPv6(Internet protocol version 6) network's host,prefixes,address and other such configurations.The prefix is used for sending the messages of DHCPv6 in the default situation.
In default situation the prefix acts as the cluster of the IP address and thus sends the the message to the destination using the address. Other options are incorrect because anycast address, local link and FF02::0A does not transmit the message in the default situation.Thus the correct option is router's prefix.