Which of the following is a built-in feature of IPv6 that is not mandatory in…

Concept

A capability is treated as "built into" a network-layer protocol when the protocol's own architecture defines and carries the mechanism natively, rather than leaving it to a separate add-on bolted on later. IPv4 was designed before security was a first-class concern, whereas IPv6 was specified from the start with a dedicated security architecture as part of the protocol itself.

In IPv6, IPsec — the Authentication Header (AH) and the Encapsulating Security Payload (ESP) — is carried natively as standard IPv6 extension headers, making authentication and encryption an integral element of the protocol design. In IPv4 the same IPsec is purely an optional add-on that the base IPv4 standard never assumes.

A precise note on the requirement level

Historically IPv6 node requirements specified IPsec support as a MUST (RFC 4294); later revisions (RFC 6434, RFC 8504) relaxed this to a SHOULD. So in current standards IPsec is the native, architecturally built-in security mechanism of IPv6 even though its implementation is now formally a SHOULD — which is still fundamentally different from IPv4, where IPsec is an entirely external optional extension. For an exam, the intended contrast is: native/built-in security in IPv6 versus optional add-on in IPv4.

Applying it to this question

• IPsec encryption support is the native IPv6 security mechanism (AH and ESP carried as IPv6 extension headers), whereas in IPv4 it is only an optional bolt-on — this is the feature built into IPv6 but not part of IPv4 by default.

• Every other choice is a service or technique that exists independently of the IP version and works the same way on both IPv4 and IPv6.

Why the other choices are not the distinguishing feature

• NAT is an address-translation workaround created for IPv4 address scarcity; IPv6's huge address space is meant to remove the need for it, so it is not an IPv6 built-in feature.

• DHCP is an external address-assignment service that runs over both versions (DHCPv6 for IPv6); it is not unique to IPv6 and is not part of the IP layer itself.

• DNS is an application-layer name-resolution system shared by both versions; it is unrelated to what is built into the IPv6 protocol versus IPv4.

• Static routing is a manual routing-configuration method available on any IP network regardless of version, so it does not distinguish IPv6 from IPv4.

Therefore the capability that is native to IPv6 but only an optional add-on in IPv4 is IPsec encryption support.