Which service abstracts away the management of the operating system, middleware, and runtime?

IaaS (Infrastructure as a Service) provides virtualized compute, storage, and networking, but the customer is still responsible for managing the OS, middleware, and runtime.

Platform as a Service (PaaS) sits between IaaS and SaaS in the cloud responsibility model. The provider manages the underlying infrastructure, operating system, middleware, and runtime environment, so the customer only needs to deploy and manage their applications and data.

SaaS (Software as a Service) delivers a fully managed application to end users and abstracts even more than PaaS, but the question specifically describes the PaaS responsibility boundary.

BMaaS (Bare Metal as a Service) provides dedicated physical hardware with no virtualization abstraction, placing full OS and software management on the customer.