Which of the following is not a recommended architecture?

The architecture that is not recommended is one that utilizes multiple environments on a single host. This setup can lead to significant challenges concerning resource allocation, performance, and security. By running multiple environments on a single host, you risk resource contention where one environment could adversely affect the performance or stability of others due to shared CPU, memory, and storage resources. Additionally, this configuration could present security vulnerabilities, as a compromise in one environment might facilitate access to other environments on the same host.

In contrast, a single application server may provide a streamlined approach for smaller deployments, and having separate user and database firewalls is a recommended security best practice, enhancing the protection of sensitive data and resources. A two-tier system, while typically less scalable, can be suitable for certain applications with specific use cases. Therefore, the recommendation of having multiple environments on a single host is generally seen as risky and ineffective, making it an ill-advised architectural choice.