The last step in the incident response methodology, according to the Network Defender (CND) guidelines, is a follow-up. This step is crucial as it involves reviewing and analyzing the incident to understand what happened, how it was handled, and how similar incidents can be prevented in the future. It includes updating incident response plans, improving security measures, and providing training to prevent future incidents.

Selective Acknowledgment (SACK) is one of the forms of acknowledgment. With selective acknowledgments, the sender can be informed by a data receiver about all segments that have arrived successfully, so the sender retransmits only those segments that have actually been lost. The selective acknowledgment extension uses two TCP options: The first is an enabling option, "SACK-permitted", which may be sent in a SYN segment to indicate that the SACK option can be used once the connection is established. The other is the SACK option itself, which can be sent over an established connection once permission has been given by "SACK-permitted".
Answer option A is incorrect. Block Acknowledgment (BA) was initially defined in IEEE 802.11e as an optional scheme to improve the MAC efficiency. IEEE 802.11n capable devices are also referred to as High Throughput (HT) devices. Instead of transmitting an individual ACK for every MPDU, multiple MPDUs can be acknowledged together using a single BA frame. Block Ack (BA) contains bitmap size of 64*16 bits. Each bit of this bitmap represents the status (success/failure) of an MPDU.
Answer option B is incorrect. With Negative Acknowledgment, the receiver explicitly notifies the sender which packets, messages, or segments were received incorrectly that may need to be retransmitted.
Answer option C is incorrect. With Cumulative Acknowledgment, the receiver acknowledges that it has correctly received a packet, message, or segment in a stream which implicitly informs the sender that the previous packets were received correctly. TCP uses cumulative acknowledgment with its TCP sliding window.