Which of the following tools helps security professionals test a web application's security?

The most suitable choice for testing a web application's security from the provided options is Acunetix. This tool is designed specifically for scanning web applications to identify vulnerabilities such as cross-site scripting (XSS), SQL injection, and other common security flaws.

Acunetix automates the process of testing web applications by performing extensive scans to uncover potential weaknesses, providing security professionals with detailed reports and recommendations for mitigation. Its ability to quickly analyze web applications for a wide range of vulnerabilities makes it a preferred choice among security professionals.

While SQLMap is also a powerful tool that focuses specifically on detecting and exploiting SQL injection vulnerabilities, it does not provide the comprehensive web application security analysis that Acunetix offers. AppScan is a well-known application security tool, but Acunetix is often recognized for its user-friendly interface and tailored focus on web application vulnerabilities. Watcher, on the other hand, does not have the same level of recognition or capability as the other tools listed for web application security testing.