The correct order reflects the logical sequence of operations a next-generation firewall (NGFW) performs on a packet.

1. DoS Protection: This is the first step. The firewall immediately checks for and mitigates Denial of Service (DoS) attacks at the ingress stage to protect its own resources and the internal network before any further processing is done.
2. Security Policy Lookup: Once the packet passes initial checks, the firewall consults its security policies to determine if the session is allowed based on attributes like source, destination, and port. If the policy denies the traffic, the packet is dropped without wasting resources on content inspection.
3. Content Inspection: For traffic that is permitted by the security policy, the firewall then performs deep packet inspection. This involves identifying the application (App-ID) and scanning the content for threats, malware, and other vulnerabilities (Content-ID).
4. QoS Shaping Applied: This is the final step, occurring at the egress stage. After a packet has been fully inspected and approved for forwarding, Quality of Service (QoS) policies are applied to manage bandwidth and prioritize the traffic as it leaves the firewall.

Palo Alto Networks. (2021). PAN-OS® Administrator’s Guide Release 10.1. "Packet Flow Sequence in PAN-OS". This official documentation details the life of a packet. It confirms that Zone Protection (which includes DoS protection) occurs at the ingress stage (Flow lookup), followed by security policy evaluation during session setup (Slow Path), then content inspection, and finally QoS shaping at the forwarding/egress stage.

Stallings, W. (2017). Cryptography and Network Security: Principles and Practice (7th ed.). Pearson. In discussions on firewall processing (Chapter 21, "Firewalls"), the book outlines a general model where initial packet filtering and connection state checks precede the application of more complex security rules and deep content inspection. Egress traffic management like QoS is logically the final step.