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?
commensalism
mutualism
parasitism
competition

The answer is mutualism.

Mutualism is a relationship between two different species in which both species benefit. Termites live feed on dead plant materials and cellulose. But their body cannot produce enzymes responsible for cellulose degradation. So, they established symbiotic interactions with gut bacteria. Gut bacteria break down carbohydrates, such as cellulose to the smaller units, which are then absorbed and oxidized by the termites. On the other hand, the bacteria depend on those nutrients.

Commensalism is a relationship of two species in which only one of them has benefit, and the other one is not affected. Parasitism is a relationship of two species in which only one of them has benefit, and the other one is harmed. Competition is a contest between two different species or between organisms inside one species.

The answer is  mutualism.