To choose a suitable machine type for a monolithic application running on Compute Engine, what method should you use?

Using the Recommender API to apply recommendations is the most effective method for selecting a suitable machine type for a monolithic application running on Compute Engine. The Recommender API analyzes the usage patterns of your resources, taking into account historical performance metrics and current workload requirements. It provides tailored suggestions for machine types that can optimize performance and cost, ensuring that your hardware resources align with the specific needs of your application.

In contrast, creating an Agent Policy for Ops Agent installation focuses more on configuration management and monitoring rather than selecting the appropriate machine type. Installing the Ops Agent manually via the gcloud CLI is also not directly related to choosing a machine type; it revolves around monitoring and managing your Compute Engine instances. Similarly, while reviewing the Cloud Monitoring dashboard for CPU utilization can provide insights into your application's performance over time, it does not offer proactive management or recommendations to determine the optimal machine type that balances performance and cost based on usage patterns. Thus, utilizing the Recommender API is the best approach for this purpose.