Choice "B" is correct. Extranets are networks that directly link certain intranets of an organization with other organizations or individuals with which the organization may have a relationship. Extranets can enable customers, vendors, suppliers, consultants, subcontractors, and others to gain access to certain areas of a private intranet.
Choice "a" is incorrect. Extranets cannot be accessed by just any person on the Internet. In order to gain access to an extranet, permission and a password are generally needed.
Choice "d" is incorrect. Extranets are not backup intranets.
Choice "c" is incorrect. Extranets are a specific area of an intranet; extranets are not software.