Infrastructure as a Service (IaaS) delivers fundamental computing resources over the cloud. These include virtual machines, block storage, networking, and load balancers. Customers use these resources to build and manage custom IT solutions, while the provider manages the underlying hardware.
PaaS abstracts infrastructure further, providing a development environment for applications without requiring infrastructure management. SaaS delivers fully functional applications over the internet. NaaS is a narrower category focusing on network delivery.
IaaS is the correct answer because it gives maximum flexibility and control compared to the other models, allowing organizations to build tailored environments. It also requires customers to manage operating systems, middleware, and runtime security, making shared responsibility an essential part of the model.