Final answer:
The docker command to add or update placement constraints is docker service update --constraint-add [constraint] [service ID], with the correct answer being option b) from the provided choices.
Step-by-step explanation:
The correct docker command to add or update placement constraints for a service is: docker service update --constraint-add [constraint] [service ID]. Therefore, the correct answer is option b). When you want to update an existing service to change its constraints, you use the docker service update command along with the --constraint-add flag to add new constraints or --constraint-rm to remove existing ones. This enables you to specify where the service should be run within a swarm, based on node labels or other criteria.
For example, to add a constraint that the service should only run on nodes with the label 'role=worker', you would use:
docker service update --constraint-add 'node.role==worker' my-service
Where 'my-service' is replaced by your actual service ID.