Structures of four disaccharides are given below. Among the given disaccharides, the non-reducing sugar is :

Step-by-step solution

Structure (3) given is of sucrose, which is a non-reducing sugar. For a sugar to be non-reducing, it must have an acetal linkage (not a hemiacetal linkage). In sucrose, both the anomeric carbons of glucose and fructose are involved in the glycosidic bond, so there is no free anomeric carbon available to open and act as a reducing agent. The other three structures have at least one free hemiacetal group, making them reducing sugars. Hence, the non-reducing sugar is structure (3).