Which cloud service model retains overall infrastructure security under the provider's management?

In the context of cloud service models, Platform as a Service (PaaS) is the correct answer because it allows developers to build, deploy, and manage applications without worrying about the underlying infrastructure. In PaaS, the cloud service provider is responsible for maintaining and securing the overall infrastructure, which includes hardware, networking, and operating systems. This model abstracts the complexities of managing the infrastructure from the user, allowing them to focus solely on application development and deployment.

The infrastructure managed by the provider ensures that it is secure against potential vulnerabilities, while also allowing users to utilize the platform's development tools and environments. As a result, users benefit from the provider's efforts to keep infrastructure security robust without needing to directly manage those aspects themselves.

In contrast, Infrastructure as a Service (IaaS) places more responsibility on the user to manage security for the operating systems, applications, and network configurations they deploy, while Software as a Service (SaaS) typically involves the provider managing all aspects concerning the applications themselves but not necessarily the underlying infrastructure. A hybrid cloud is a combination of private and public clouds and does not exclusively fit the criteria for this question, as it could involve shared responsibilities for security depending on the configuration.