a) Software as a Service (SaaS)
SaaS refers to full applications deployed in the cloud, whereas PaaS and IaaS refer to cloud services that allow for development of new applications with varying levels of hardware abstraction. IaaS usually refers to cloud Virtual Machines, and it allows more control over the types of computers used to run an application (Linux/Windows/RAM/CPUs/etc). PaaS is a set of minimum required components to make an application run, typically not visible to developers. Developers can write code and deploy to a PaaS service without configuring virtual machines or installing dependencies/Operating Systems.