The type of computer networking accessible to computers that are not physically part of an organization's own private network but are also not accessible to the general public is

An extranet extends limited access to outside partners who are neither inside the private network nor part of the general public.

The internet is the fully public global network, open to everyone, unlike the described restricted access.

An intranet is entirely internal to one organisation, closed to outsiders such as partners.

No other choice fits because extranet exactly names this partner-facing network.

An extranet often uses secure logins so selected suppliers or clients can reach shared resources.

Extranets commonly run over the internet protected by authentication and encryption.