From an organic chemistry perspective, you can consider protecting groups. If you recall from carbonyl chemistry, ketals (RO-C-OR) are the protected form of ketones. They are inert to many reactions. That can be applied to the sugar linkage. In choices A, B, and C, you'll notice that the right sugar (ß-D-glucopyranose in each case) is in its hemiacetal state (the right side of the right sugar is RO-C1-OH), which is not protected. It can undergo oxidation (converting Ag+ into Ag(s), forming the "silver mirror" in a Tollen's test), which is why they are referred to as reducing sugars.
In sucrose (choice D), it is a linkage between the anomeric carbon (C1) of glucose and the anomeric carbon (C2) of fructose. This results in a ketal linkage (rather than hemi), so the sugar cannot be oxidized (is not a reducing sugar). In essence, it is protected.
If you happen to have a BR organic book, then look at Book 2 pages 106 and 110.
