Which of these is not a cloud deployment model?

The correct choice identifies "System administration as a service" as not being a recognized cloud deployment model. In the context of cloud computing, deployment models refer to the specific ways resources and services are delivered over the internet.

Infrastructure as a Service (IaaS), Platform as a Service (PaaS), and Software as a Service (SaaS) are well-established categories within the cloud ecosystem, each offering different levels of control, flexibility, and management, reflecting varying degrees of abstraction and responsibility for the user.

IaaS provides virtualized computing resources over the internet, giving users the ability to manage servers and storage while the cloud provider manages the physical infrastructure. PaaS offers a platform allowing customers to develop, run, and manage applications with the underlying architecture handled by the provider. SaaS delivers software applications over the internet, enabling users to access them typically via a subscription model without the need for local installation.

In contrast, "System administration as a service" is not a standard or widely accepted deployment model in cloud computing. It doesn’t fit within the established frameworks of delivering specific services and resources, thus confirming it as the outlier in this set of options.