The command docker pull ubuntu retrieves the latest version of the Ubuntu image from the default Docker hub registry. docker pull ubuntu is the correct command because it specifies only the image name, which defaults to pulling the 'latest' tag if no other version is specified. pull ubuntu is incorrect because it is missing the docker command. docker push ubuntu is incorrect because it is using push which is for uploading images, not downloading them. docker rmi ubuntu is incorrect because rmi is used to remove images, not to pull them.