Concept:The rumen is a large fermentation chamber where fibrous feed is digested with the help of microbes.Explanation:Ruminants cannot digest cellulose on their own because they lack the necessary enzymes.Micro-organisms such as bacteria and protozoa live in the rumen and produce enzymes that break down cellulose and other fibrous materials.These microbes ferment the feed into substances the animal can absorb and use for energy.Therefore, the presence of these micro-organisms enhances the digestive function of the rumen.The other options are not the main reason for enhanced rumen digestion.Answer:B. Presence of micro-organisms