Which service model provides users with the least control over the underlying infrastructure?

The service model that provides users with the least control over the underlying infrastructure is Software as a Service (SaaS). In the SaaS model, users access software applications hosted on the service provider's infrastructure over the internet. The service provider manages all aspects of the application, including maintenance, updates, and security, which means users have very little influence over how the infrastructure operates.

This level of abstraction allows users to focus on utilizing the software rather than managing the underlying systems and resources. As a result, they cannot customize the hardware or software environment to fit specific operational needs, which is a stark contrast to Infrastructure as a Service (IaaS) or Platform as a Service (PaaS), where users have more control and freedom to configure resources according to their requirements.

In IaaS, users can control the virtual machines and networking components, while PaaS allows greater control over application development and deployment but still abstracts some layers of management. Hybrid cloud is not a service model on its own but rather a combination of cloud and on-premises resources, allowing organizations to maintain control over some elements while leveraging cloud services for others.