Which cloud service model allows outsourcing of the infrastructure to a service provider?

Infrastructure as a Service (IaaS) is the correct choice because it specifically pertains to the outsourcing of physical and virtual infrastructure such as servers, storage, and networking to a cloud service provider. With IaaS, users are provided with on-demand computing resources that they can configure and manage remotely, allowing them to scale their infrastructure needs as required without the need for physical hardware management.

This model is designed to provide extreme flexibility and scalability, as it permits organizations to avoid the complexities of owning and maintaining their infrastructure, instead focusing on utilizing the services as needed. The service provider is responsible for managing the underlying physical resources, which relieves the client of the burden related to procurement, setup, and maintenance.

In contrast, Software as a Service (SaaS) delivers complete software applications that users can access over the internet, while Platform as a Service (PaaS) offers environment and tools for developers to build applications without managing underlying hardware or software layers. Network as a Service (NaaS) focuses specifically on delivering networking capabilities over the internet, rather than providing full infrastructure solutions. Thus, IaaS stands out as the model explicitly tailored for infrastructure outsourcing.