The most efficient search would focus on minimizing the data processed at each step. Search option C: 'index=foo sourcetype=bar | fields host | lookup mylookup host OUTPUT host_flag | where host_flag="true" | stats count by host' is the most efficient. This is because it uses the 'fields' command to limit the data to only the 'host' field before applying the lookup and further processing. This reduces the amount of data processed by the lookup and subsequent commands, making it more efficient.