Which protocol can an Internet browser use to download the PAC file with the web proxy configuration?

While HTTPS can be used for secure delivery, HTTP is the most common and standard protocol for PAC file download, especially for initial discovery.

FTP is a file transfer protocol but is not the standard protocol browsers use to automatically discover and download PAC files.

TFTP is a simple file transfer protocol typically used for network device configuration and is not used by browsers for PAC files.

Web browsers are designed to fetch PAC files via HTTP because it is a standard web protocol for delivering such configuration scripts, often specified by the browser's proxy settings.