An application-level firewall is the most effective in detecting information hiding in TCP/IP traffic.
Information hiding is a technique that conceals data or messages within other data or messages, such as using steganography, covert channels, or encryption. An application-level firewall is a type of firewall that operates at the application layer of the OSI model, and inspects the content and context of the network packets, such as the headers, payloads, or protocols. An application-level firewall can help to detect information hiding in TCP/IP traffic, as it can analyze the data for any anomalies, inconsistencies, or violations of the expected format or behavior. A stateful inspection firewall, a content-filtering proxy, and a packet-filter firewall are not as effective in detecting information hiding in TCP/IP traffic, as they operate at lower layers of the OSI model, and only inspect the state, content, or header of the network packets, respectively. References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 6: Communication and Network Security, page 731; Official (ISC)2 Guide to the CISSP CBK, Fifth Edition, Chapter 4: Communication and Network Security, page 511.