Which system consists of a publicly available set of databases that contain domain name registration contact information?

IANA (Internet Assigned Numbers Authority) coordinates global IP address allocation and protocol parameter registries but does not maintain domain registration contact databases.

CAPTCHA is a challenge-response mechanism designed to distinguish human users from automated bots and has no relation to domain registration data.

IETF (Internet Engineering Task Force) develops and publishes internet standards and RFCs but does not operate or maintain domain registration contact databases.

WHOIS is a TCP-based query/response protocol defined in RFC 3912 that provides access to publicly available databases containing domain name registration details including registrant name, organization, address, and contact information. It is a standard reconnaissance tool used by security professionals to gather ownership data about target domains during the information-gathering phase of an assessment.