Termites are unable to digest some plant materials (fiber). termite stomachs contain bacteria that digest the fiber in wood. which type of relationship does this describe?

The answer is a symbiotic relationship.

Symbiosis is an interaction between two different species. Termites live feed on dead plant materials and cellulose. But their body cannot produce enzymes responsible for cellulose degradation. Therefore, they established symbiotic interactions with gut bacteria which are able to digest the fiber in the wood. Gut bacteria break down carbohydrates to the smaller units, which are then absorbed and oxidized by the termites