The correct answer is B because the Web Application Firewall is itself a primary evidence source during web-attack investigations, especially when the incident involves bot traffic, injection attempts, and application-facing abuse. CHFI v11 explicitly includes WAF fundamentals, benefits and limitations of WAF, and web-application forensics involving collection and analysis of multiple log sources. In methodology terms, when investigators are assembling evidence for a web attack, they are expected to gather logs not only from web and application servers, but also from defensive controls such as the WAF because those systems often capture blocked requests, signatures triggered, payload patterns, source information, and timestamps. The other options are not evidence-collection steps focused on the gateway. Tracing the attacking IP is an attribution task that usually comes later, encrypting and checksumming logs is an integrity-preservation measure rather than the collection step itself, and forensic imaging is aimed at storage media rather than gateway log evidence. Because the question asks which methodology step explicitly includes output from the software-based gateway, the correct answer is to collect WAF logs.