The DHCPv6 Guard feature blocks DHCP reply and advertisement messages that originate from unauthorized DHCP servers (-> Answer D is not correct) and relay agents that forward DHCP packets from servers to clients. Client messages or messages sent by relay agents from clients to servers are not blocked (-> Answer C is not correct) Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6_fhsec/configuration/xe-16/ip6f- xe-16-book/ip6-dhcpv6-guard.html