Final answer:
To deploy a legacy app with specific configuration and OS requirements, virtualization is the most suitable approach because it can isolate the application in a virtual machine with necessary settings, unlike Docker which requires compatible OS.
Step-by-step explanation:
When deploying and supporting a legacy application with very specific OS and configuration requirements that cannot be changed, the best approach is often to use virtualization. This allows you to create a virtual machine (VM) that can run the specific OS and configuration needed for the application. Docker containers might be an alternative if the application and its dependencies are well encapsulated and the OS requirements are compatible with containerization. However, using a virtual environment ensures that the legacy application can operate in a consistent and isolated manner, preventing any updates or changes in the host system from affecting the application's functionality.