Final answer:
To allow developers to focus on writing custom code without managing the underlying infrastructure, Platform as a Service (PaaS) is the recommended cloud service, as it eliminates the need for them to handle operating systems, runtimes, and middleware.
Step-by-step explanation:
The client requires a development platform where the developers can write business requirement-oriented custom code without having to manage operating systems, runtimes, and middleware. The most suitable cloud service offering for this requirement is Platform as a Service (PaaS). PaaS provides a platform allowing customers to develop, run, and manage applications without the complexity of building and maintaining the infrastructure typically associated with developing and launching an app.
Infrastructure as a Service (IaaS) provides virtual machines and other abstracted hardware and networking resources, but the clients would still need to manage the OS and runtimes. Function as a Service (FaaS) is a more event-driven and execution-based model where code is executed in response to events, which might be too granular for the client's needs. Software as a Service (SaaS) delivers software over the Internet, which doesn't fit the requirement of developing custom code. Therefore, PaaS is the recommended solution.