Google Cloud Certified Associate Cloud Engineer Practice

Service Accounts are used for authenticating between different services in Google Cloud. Service accounts are like special users that are created for different services to communicate with one another. Pub/Sub is a messaging service designed for asynchronous communication between services. Stackdriver is a monitoring service used for identifying, tracking, and reporting on cloud resources. Compute Engine, or virtual machine instances, can use Container-Optimized OS (COS) to run containers, but this is not specific to authentication between services. Therefore, A (Service Accounts) is the most appropriate and specific answer.