In which service model allows users to install all software on vendor-provided hardware?

The correct service model that allows users to install all software on vendor-provided hardware is Infrastructure as a Service (IaaS). In this model, the cloud provider supplies the underlying infrastructure, including virtual machines, storage, and networking, while users have the flexibility to install their own operating systems and applications on that infrastructure. This level of control enables users to fully customize their software environment to meet specific needs, as they are essentially managing and controlling their own software stack.

In contrast, Software as a Service (SaaS) delivers software applications over the internet on a subscription basis, where the service provider manages everything from infrastructure to applications. Users cannot install their own software in this model; they can only utilize the applications provided by the vendor.

Platform as a Service (PaaS) provides a platform that includes infrastructure and middleware, allowing developers to build, deploy, and manage applications without worrying about the underlying hardware. While it offers more flexibility than SaaS, it does not provide the same level of control over the software stack as IaaS does.

Understanding these distinctions helps clarify why IaaS is the appropriate answer, as it specifically allows full control over the software installations on the hardware controlled by the vendor.