Which of the following is an open source Web scanner?

Nikto is a free, open source command-line web server scanner released under the GPL license and maintained on GitHub. It performs comprehensive tests including checks for over 6700 potentially dangerous files, outdated server versions, and version-specific problems. Its open source nature and web-application focus make it the correct answer.

GFI LANguard is a commercial network security scanner focused on patch management and network auditing, not an open source web scanner.

NetRecon was a commercial network vulnerability scanner developed by Symantec and is neither open source nor web-specific.

Internet Scanner was a commercial vulnerability assessment product from IBM ISS and is not open source.