Answer
Linux
Step-by-step explanation
A website is a collection of related web pages, including multimedia content, typically identified with a common domain name, and published on at least one web server. To host a website one needs an operating system which is system software that manages computer hardware and software resources and provides common services for computer programs.
So if Dave wants to host a website and he needs an operating system that is free of cost he should use LINUX operating system. Which is an open source operating system. Linux OS, is a freely distributable, cross-platform operating system based on Unix that can be installed on PCs, laptops, notebooks, among others.