Hint: Digestion in human beings is of two types - mechanical and chemical digestion. Mechanical digestion takes place in the mouth and small intestine. When a food item is ingested, the chemical digestion starts in the mouth by the help of saliva. This bolus is then pushed inwards through the pharynx into the oesophagus, this process is called swallowing. Food then enters the stomach via the oesophagus.
When the food enters the stomach, G cells of the stomach mucosa secretes a hormone called gastrin. It stimulates the secretion of gastric juice by gastric glands. Then the food enters the small intestine. Here, the chemical as well as mechanical digestion takes place. When the food leaves the stomach through its pyloric end and enters the duodenum, it is called chyme.
The major digestion occurs in the small intestine. The large intestine plays a very minimal role in the digestion process. It is responsible for the absorption of some water and certain drugs. The complete chemical digestion in the human body is mediated by various enzymes. The food rich with cellulose is not digested by the digestive system of the human body. There are no enzymes present for the cellulose digestion present inside the body.
The molecular structure of cellulose is composed of linear polysaccharide units different from the starch. Thus the enzymes for starch digestion are present but for cellulose digestion, there is no enzyme present inside our body.
Hence, the correct answer is option (D).
Note: The digestion of food material in the human body is a sequential process. After the digestion completes, the absorption of the final digestive end products has to take place. It occurs through the intestinal mucosa into the blood. It is carried by the passive, active or facilitated transport. After the absorption process is completed, assimilation and the egestion takes place.