Final answer:
Neighbor Solicitation messages are multicast as part of the Duplicate Address Detection process in IPv6 to test for duplicate addresses. If a device receives a response, it means the address is already in use, otherwise, the address is unique and can safely be assigned.
Step-by-step explanation:
Neighbor Solicitation can be used to test for duplicate IPv6 addresses through option C) By sending a multicast Neighbor Solicitation message. When a device is joining an IPv6 network, it generates a link-local address using its MAC address with the FE80::/10 prefix and the EUI-64 format. To ensure that this address is unique on the local link, it sends out a Neighbor Solicitation message as part of the Duplicate Address Detection (DAD) process.
The Neighbor Solicitation message is sent as a multicast to the solicited-node multicast address that corresponds to the address being verified. If another device on the network is already using the same address, it will respond with a Neighbor Advertisement indicating that the address is in use. In the absence of a response, the address is considered unique and can be used by the device. This process is a fundamental part of the IPv6 protocol suite and helps prevent duplicate address conflicts in the network.