What does the Domain Name Service return when a domain is looked up?

The Domain Name Service (DNS) is a system that translates human-readable domain names, like www.example.com, into numerical IP addresses that computers use to identify each other on the network. When a domain is looked up, the primary function of DNS is to return the corresponding IP address for that domain. This is crucial because while users may prefer to remember website names, the network infrastructure operates on IP addresses.

The other options do not represent what DNS directly returns. A server refers to the hardware or software that provides services, while a host can refer to any device connected to a network. A document typically refers to web content, such as HTML pages, which is served to a browser once the correct IP address has been determined. Therefore, the appropriate response in this context is the IP address, as it is the specific information that DNS provides during a domain lookup.