A filter must be _____ when a head loss of 6 to 10 feet occurs.

When a filter experiences a head loss of 6 to 10 feet, it is indicative of a significant buildup of particles and contaminants within the filter medium, which can impede the flow of water and reduce filtration effectiveness. Backwashing is the process of reversing the flow of water through the filter to dislodge trapped solids and clean the filtration medium. This action restores the filter's operational efficiency by removing accumulated debris that causes head loss.

In this context, cleaning might imply a manual or thorough cleaning process, which may not be necessary at this point, and replacement would be considered a more extreme measure that is not typically required unless the filter media is severely compromised. Inspection is important, but it is more of a proactive or periodic measure rather than a direct response to high head loss. Therefore, backwashing is the appropriate action to take in response to head loss within the specified range.