What cloud service allows subscribers to use fundamental IT resources on demand?

Infrastructure-as-a-Service (IaaS) is the correct answer because it provides users with fundamental IT resources such as virtual machines, storage, and networks over the internet on a pay-as-you-go basis. This service model allows organizations to access scalable and flexible computing resources without having to invest in physical hardware. Subscribers can manage and configure the underlying infrastructure to meet their specific needs, enabling higher efficiency and cost savings.

In contrast, Platform-as-a-Service (PaaS) offers a framework for developers to build applications without dealing with the underlying infrastructure, which is more focused on application development rather than providing raw IT resources. Software-as-a-Service (SaaS) delivers software applications over the internet, where users simply interact with software hosted on external servers, again not focusing on IT resource provisioning. Thus, while each service model has its unique advantages, IaaS specifically targets on-demand access to essential IT resources.