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

IANA (Internet Assigned Numbers Authority) manages global IP address allocation, port numbers, and protocol parameters - it does not serve as a public database of domain registration contact information.

CAPTCHA is a challenge-response test used to distinguish humans from bots and has no relation to domain name registration databases.

IETF (Internet Engineering Task Force) is the standards body that develops internet protocols through RFCs, not a database containing domain registration contact information.

WHOIS is a query/response protocol defined in RFC 3912 that provides publicly available information about domain name registrants, including contact details, registration dates, and name servers. It is maintained by registrars and regional internet registries and is commonly used during the reconnaissance phase of penetration tests and investigations. It directly matches the description of a publicly available set of databases containing domain name registration contact information.