Which of the following is a validly formed fully qualified domain name (FQDN)?

A fully qualified domain name (FQDN) consists of a hostname and a domain name, which can include multiple hierarchical levels, ending with a top-level domain (TLD). In this case, "jefferson.lib.co.us" is a validly formed FQDN because it adheres to the proper structure. It includes a subdomain "jefferson," the second-level domain "lib," and the third-level domain "co," culminating in the country-code top-level domain "us."

In contrast, "example.com" is valid as well but does not illustrate the use of multiple hierarchical levels. "test@domain.com" is not an FQDN; it utilizes an email format instead. Similarly, "www!example.org" fails as it contains an invalid character (the exclamation mark), which is not allowed in domain names. Thus, "jefferson.lib.co.us" is the clear example of a proper FQDN structure.