The digestive system of Humans has many enzymes, and acids to break down and digest all kinds of different foods (carbohydrates. proteins and fats), but do not have the enzymes needed to break down cellulose. Cows, Goats, Sheep and other animals that graze on many organic foods(For example, grass) do have the enzymes needed to break down cellulose. Hence The part of food that is not digested in the body is Cellulose because the cellulose-digesting enzyme is absent.
So, the correct answer is 'cellulose'.