Which cloud model is characterized by sharing resources that reside outside of user firewalls?

The correct choice is the public cloud model, which is characterized by resources that are shared and reside outside of user firewalls. In a public cloud, services and infrastructure are made available to multiple clients over the internet, allowing for shared resources such as storage, applications, and computing power. This model provides scalability, cost-effectiveness, and ease of access, as users can leverage the cloud provider's infrastructure without the need for extensive on-premises resources.

The essence of a public cloud lies in its multi-tenant architecture, meaning that various users share the same physical infrastructure while remaining isolated in terms of their data and applications. This shared environment can lead to significant cost savings and operational efficiencies, as clients can simply pay for the resources they consume without having to invest in dedicated hardware or maintenance.

In contrast, the private cloud model involves dedicated resources that are used exclusively by one organization, typically hosted within that organization’s own data center or managed by a third party, and therefore does not share resources with others outside of its firewall. A community cloud is more collaborative, used by multiple organizations with shared concerns, while a hybrid cloud combines both public and private clouds but still maintains distinct resource boundaries, thus not aligning with the definition presented in the question.