The router sends its prefix and then the node adds its interface ID to make its own IPv6 address, thus Answer A is correct.
The following is the actual procedure for IPv6 stateless address autoconfiguration:
(1) The new node on the network generates link local address and allocates it to the interface. Link-local address takes the following form: fe80:0000:0000:0000:0000:
(2) The node confirms that generated link local address is not already used on the same network (DAD: Duplicate Address Detection). At first, the node transmits Neighbor Solicitation (NS) message on the network. If another node is already using the same address, this node sends Neighbor Advertisement (NA) message. The new node that transmitted NS message will use the original link-local address if it receives no NA message after a certain time. If the new node is notified of the duplicate address situation, it will not allocate the link-local address and terminates the interface.
(3) The new node sends Router Solicitation (RS) message on the network to request information, using the link-local address just allocated. RS message transmission is not a must. The node can passively wait for (4).
(4) The node that received RS message (usually a router) sends back Router Advertisement (RA) message. RA message is transmitted periodically, so nodes do not necessarily have to send RS message.
(5) The node receives RA and gets IPv6 address prefix.
(6) The node forms the global IPv6 address by combining prefix and interface ID, just as it did for link-local address.
It should be noted that RA sender, such as router, only sends fixed prefix allocated to the network. In other words, RA sender does not care to whom it sent information. It does not maintain such records. Therefore, if two routers exist on the same network and advertise different prefixes with RAs, receiving node automatically gets both RA to allocate different address on the same interface
