Option A is correct because filtering the Activity log by the Configuration category and then by Service Account as the Resource type will display the creation event of the service account. This method directly targets the information needed to ascertain when the service account was established. Options B and D are incorrect as they involve filtering by Google Project, which does not provide the specific details about the service account's creation. Options C and D are also incorrect because the Data Access category does not include creation events for service accounts. For more detailed information, refer to Google Cloud's documentation on activity logs and audit logs.