Final answer:
The correct command to display logs for a specific service in a Docker Swarm is 'docker service logs service_name'. This provides the log output from all tasks of the service.
Step-by-step explanation:
The command used to display the logs for a specified service in a Docker Swarm is B) docker service logs service_name. Using this command, you can retrieve the log output from all the tasks of the service.
If you only want to see the logs from a specific task or tasks, you can specify it by appending the task ID to the service name.