Final answer:
To remove a 'down' node from a Docker swarm cluster, the correct command is 'docker node rm -f'. Running this command should be done from a manager node, and will help to manage the swarm by redistributing tasks previously assigned to the removed node.
Step-by-step explanation:
To remove a 'down' node from a Docker swarm cluster, you would generally use the command docker node rm or docker node remove, optionally with the -f flag if you need to forcefully remove a node that is unresponsive or in a 'down' state.
Specifically, the correct command from the provided options would be B) docker node rm -f. Please note that to run this command, you need to be operating from a manager node in the swarm. If you are on the node that needs to be removed, and it is still operational, you could use the command docker swarm leave. After a node is removed, the swarm will attempt to reconcile and redistribute tasks that were running on the removed node to other nodes in the cluster.