Mutualism can be defined as a type of symbiotic relationship.
Symbiosis can be defined as the type of ecological interplay in between two species localized in the same habitat in which at least one of the species gets benefited in the form of food, shelter or protection due to the interaction.
Mutualism is a type of positive symbiosis in which both the species involved in the ecological interplay benefits due to the interaction in between them.
In the given question, some micro-organisms survive in the digestive tract of the cattle.
These micro-organisms produce the enzyme, cellulase, which helps in digesting cellulose present in the food consumed by the cattle.
The cattle is incapable of producing this cellulase enzyme by itself and is dependent on the micro-organisms for its synthesis.
Hence, the the micro-organisms help the cattle in obtaining nutrition and energy by the digestion of cellulose.
On the other hand, the micro-organisms get shelter, nutrition and protection in the gut of the cattle.
