Which is a characteristic of carpenter ant tunnels?

Carpenter ant tunnels are characterized by their smooth sides. This smoothness occurs because carpenter ants do not ingest the wood they excavate; instead, they chew through it, which leaves behind polished, smooth surfaces inside the tunnels. The tunnels can often be extensive and are typically found in damp or decaying wood where the ants prefer to establish their nests.

The other options describe characteristics not typical of carpenter ant tunnels. For instance, tunnels filled with dust and frass pellets are more commonly associated with termites, which produce a mixture of digested wood and feces. Very rough sides might imply that the material was improperly excavated, which is not the case with carpenter ants. Mud tubes are specifically built by termites as protective tunnels leading to their food sources, and therefore wouldn't be found in carpenter ant structures.