Final answer:
The most suitable tool for diagnosing erratic behavior in a web application after a security patch is an HTTP interceptor, which allows for detailed analysis of HTTP/HTTPS traffic.
Step-by-step explanation:
The issue with the web application behaving erratically after a security patch indicates there might be unexpected interactions or bugs introduced by the update. In this scenario, the most appropriate tool to help identify the issue is an HTTP interceptor. This tool allows the user to view and manipulate the HTTP/HTTPS traffic between the client and the server to pinpoint problematic requests and responses. Different tools provide varying levels of interaction, but using an HTTP interceptor, such as Burp Suite or OWASP ZAP, enables a detailed examination of how the application processes requests and what might be causing the erratic behavior.
While a fuzzer could be used to identify potential vulnerabilities by sending random or malformed data to the application, it is less suited to diagnosing issues post-update as it is more for stress-testing and finding new vulnerabilities. A SCAP scanner (Security Content Automation Protocol) is used to automate the checking of system compliance with security requirements, which is not relevant here. Lastly, vulnerability scanners are designed to detect known vulnerabilities in software and would not necessarily pinpoint inconsistent behaviors due to a recent patch.