What type of machine listens for HTTP requests directed at a website's domain?

The correct choice identifies a server as the machine that listens for HTTP requests directed at a website's domain. A server is specifically designed to process requests from clients, which can include web browsers and other applications. When a user types a URL into their browser, their request travels over the internet to the server hosting that website. The server then processes the request and sends back the appropriate resources, such as HTML, images, and style sheets, to display the web page for the user.

A router, on the other hand, primarily directs traffic between different networks and does not handle HTTP requests directly. A switch connects devices within a single network in order to facilitate communication among them but does not interact with HTTP requests. A client refers to the machine or application that initiates requests to the server, rather than one that listens for those requests. Thus, the role of a server is key in managing and responding to incoming HTTP requests, which makes it the correct answer.