Which of the following fields is NOT part of the IPv4 header?

The field that is not part of the IPv4 header is the Checksum. In the context of the IPv4 header, the Checksum is actually used for the header's integrity check but is not included in the options as a field in the header structure.

To clarify, the IPv4 header includes several key fields necessary for routing and delivery, such as Version, which specifies the IP protocol version; Traffic Class, which is used for quality of service (QoS) purposes; and Time to Live (TTL), which helps prevent packets from circulating indefinitely on the network. Each of these fields plays an essential role in how packets are processed and routed through a network.

The Checksum, while important for error checking, is associated specifically with the header (and in practice, is a calculated value to ensure data integrity) rather than being an actual field within the IPv4 header itself. Therefore, among the provided options, it stands out as the correct answer to this question.