What are intranets that share a portion of their content with customers, suppliers, or other businesses, but not with the public?

The term that best describes intranets that share a portion of their content with customers, suppliers, or other businesses, while maintaining restricted access from the public, is extranets.

Extranets serve as a secure extension of an organization's intranet, allowing external entities to access specific data and resources relevant to them. This facilitates collaboration and information sharing while ensuring that sensitive internal information remains protected from public access. Organizations use extranets to enhance relationships with partners, manage supply chains, or support customer communications by providing relevant information in a controlled environment.

The other options do not align with this definition. A subnet mask pertains to network addressing and segmentation, Arpanets refers to an early packet-switching network that laid the groundwork for the internet, and Dualnet is not a standard term used in networking contexts. Therefore, extranets are the correct choice for describing those intranets that maintain security while allowing limited access to specific users outside the organization.