Answer: Nikto
Explanation:
Nikto is a web application scanner that is specifically designed to identify vulnerabilities and misconfigurations in web servers and web applications. It is known for its ability to identify the HTTP server, web applications, and various security issues related to them.
While Nessus, OpenVAS, and Arachni also have capabilities related to vulnerability scanning, Nikto is particularly focused on web server and web application vulnerabilities, making it the most likely tool being used in this scenario.