Create an allow VPC firewall rule that specifies the target/source with respective service accounts.

Answer: D

Explanation:

https://cloud.google.com/vpc/docs/firewalls#service-accounts-vs-tags

https://cloud.google.com/vpc/docs/firewalls#service-accounts-vs-tags

A service account represents an identity associated with an instance. Only one service account can be associated with an instance. You control access to the service account by controlling the grant of the Service Account User role for other IAM principals. For an IAM principal to start an instance by using a service account, that principal must have the Service Account User role to at least use that service account and appropriate permissions to create instances (for example, having the Compute Engine Instance Admin role to the project).