Which Azure service should host a legacy application that requires full operating system access and custom software installations?

Azure Virtual Machines (VMs) are the correct choice because they provide full control over the operating system, allowing for custom software installations and accommodating legacy application requirements. VMs offer infrastructure as a service (IaaS), meaning you manage the OS, runtime, and applications. Azure App Service is a platform as a service (PaaS) offering designed for web applications and APIs, with limited OS access. Azure Functions is a serverless compute service best suited for event-driven, short-running code, not full legacy applications. Azure Logic Apps is a serverless workflow service for integrating applications and services, not for hosting custom applications directly.