The IPv6 header is also called a fixed header, which contains eight fields.

The IPv6 basic header has a fixed length of 40 bytes and contains eight fields: Version, Traffic Class, Flow Label, Payload Length, Next Header, Hop Limit, Source Address, and Destination Address. The Source IPv6 Address is therefore part of the IPv6 header. Hop Limit is also present and serves a function comparable to the IPv4 TTL field by limiting how many Layer 3 forwarding hops a packet can traverse. Next Header identifies the type of extension header or upper-layer protocol that follows the IPv6 basic header. A source MAC address is not part of the IPv6 header. MAC addresses belong to the data-link-layer frame header, such as an Ethernet header, and are relevant only to the current Layer 2 segment. As an IPv6 packet is routed, Ethernet source and destination MAC addresses can change from hop to hop while the IPv6 source and destination addresses normally remain unchanged end to end. Therefore A, B, and C are IPv6 header fields, while D is not. The correct answer is A, B, and C.