As a Cloud Architect, you have to implement a CI/CD pipeline in GCP for a microservices-based application. You want to automate testing and deployment while adhering to the principles of immutability and infrastructure as code. Which combination of GCP services will best meet these requirements?

*Explanation:* To meet the requirements of automating CI/CD while adhering to immutability and infrastructure as code principles, the best choice is to use Cloud Build with Terraform. Cloud Build is well-suited for CI/CD, and Terraform is a powerful tool for managing infrastructure as code. Cloud Deployment Manager is less flexible for infrastructure as code compared to Terraform. Jenkins adds complexity and may not align with GCP best practices. Spinnaker is more complex and adds overhead that may not be necessary for your specific needs.