Which of the following diluents is known to be insoluble?

Cellulose is recognized as an insoluble diluent due to its structural properties. It is a polysaccharide that makes up the cell wall in plants and is composed of long chains of glucose units linked by β-1,4-glycosidic bonds. Unlike other carbohydrate diluents such as lactose, sucrose, and sorbitol, cellulose does not dissolve in water. Its insolubility is a key characteristic that makes it useful in various pharmaceutical and food applications where bulk is required without a high degree of solubility.

In contrast, lactose, sucrose, and sorbitol are all soluble in water and can readily dissolve to form solutions. This solubility makes them suitable for different uses but excludes them from being classified as insoluble diluents like cellulose.