Which of the following best describes a cloud service that allows immediate access to additional resources?

The term that best describes a cloud service that allows immediate access to additional resources is "on-demand service." This concept is fundamental in cloud computing, where users can provision resources as needed without delay. On-demand services enable organizations to scale their operations quickly and efficiently, responding to varying workload demands in real-time.

For instance, in a cloud environment, an application may experience fluctuating usage, requiring more computing power or storage at peak times. On-demand service allows users to tap into additional resources instantly without having to go through lengthy procurement processes or manual scaling procedures. This agility is one of the core advantages of cloud services, enabling businesses to remain flexible and responsive to their operational needs.

The other options, while related to cloud computing, do not specifically emphasize the immediacy of resource access. Infrastructure as a Service and Platform as a Service refer to different layers of cloud services but do not inherently speak to the immediacy of scaling. The hybrid cloud model describes a combination of on-premises and cloud services, which also does not convey the concept of immediate resource availability as clearly as on-demand service does.