Overview:
Lactose is a disaccharide composed of two monosaccharides, D-galactose and D-glucose, linked together by a β−1, 4- glycosidic linkage. It is a reducing sugar, meaning it has the ability to donate electrons and undergo oxidation. In this response, we will explain each of the given options and why they are true.

Explanation:

a) It on hydrolysis gives D-galactose and D-glucose:
Lactose can be hydrolyzed by the enzyme lactase to yield its constituent monosaccharides, D-galactose and D-glucose. Hydrolysis is the process of breaking down a compound using water, and in this case, it breaks the β−1, 4- glycosidic linkage between the two monosaccharides, resulting in the release of D-galactose and D-glucose.

b) It is a reducing sugar:
Lactose is a reducing sugar because it has a free anomeric carbon in the ring structure of its glucose component. The anomeric carbon can undergo oxidation and reduce other compounds. This property allows lactose to participate in various chemical reactions, such as the Maillard reaction, which gives rise to the characteristic browning and flavor development in baked goods.

c) It has β−1, 4- glycosidic linkage:
Lactose has a β−1, 4- glycosidic linkage between the anomeric carbon of D-galactose and the hydroxyl group at the fourth carbon of D-glucose. This linkage is formed through a condensation reaction, resulting in the formation of a glycosidic bond. The β configuration indicates that the hydroxyl group attached to the anomeric carbon is in the trans position relative to the hydroxyl group attached to the fourth carbon.

d) Galactose is the reducing unit in lactose:
This statement is not true. In lactose, the glucose component is the reducing unit. As mentioned earlier, lactose is a disaccharide composed of D-galactose and D-glucose. The reducing property of lactose arises from the presence of the free anomeric carbon in the glucose molecule.

In conclusion, the correct options for the given statement are A, B, and C. Lactose on hydrolysis gives D-galactose and D-glucose, it is a reducing sugar, and it has a β−1, 4- glycosidic linkage. However, it is important to note that galactose is not the reducing unit in lactose, but rather glucose.