Which one of the following statement is not true?

We have to find the statement that does not match the real structure and properties of lactose. Let us collect the accepted facts about the sugar first.

Lactose is made when one molecule of $$\beta\text{-D-galactose}$$ joins with one molecule of $$\text{D-glucose}$$ by removing a molecule of water. The removal of water creates the glycosidic bridge. Symbolically one writes

$$\text{C}_6\text{H}_{12}\text{O}_6+\text{C}_6\text{H}_{12}\text{O}_6-\text{H}_2\text{O}= \text{C}_{12}\text{H}_{22}\text{O}_{11}.$$

So lactose really has the molecular formula $$\text{C}_{12}\text{H}_{22}\text{O}_{11}.$$ Now we test the four statements one after another.

Testing option A. In lactose the anomeric carbon $$C_1$$ of galactose keeps the $$\beta$$-configuration and forms a bridge to the $$C_4$$ of the glucose unit. Chemists therefore write a $$\beta(1\!\rightarrow\!4)$$-glycosidic linkage. Option A, however, talks about an $$\alpha$$-glycosidic linkage. The given linkage description is therefore wrong.

Testing option B. After the bridge is made the anomeric carbon of the glucose molecule is still a free hemi-acetal. A free hemi-acetal can open to the aldehyde form and reduce Fehling’s reagent. Hence lactose is a reducing sugar and it does give Fehling’s test. Thus option B is a true statement.

Testing option C. We have already verified the formula $$\text{C}_{12}\text{H}_{22}\text{O}_{11}.$$ Next we count hydroxyl groups. Before condensation each monosaccharide possesses five -OH groups.

Number of -OH groups before condensation $$5+5=10.$$ During bridge formation one -OH from the anomeric carbon of galactose is lost completely and the -OH on $$C_4$$ of glucose loses only its hydrogen, so the total count of complete hydroxyl groups decreases by one: $$10-1=9.$$

However, the oxygen that remained as the bridge is no longer part of a hydroxyl; hence two complete hydroxyl groups disappear. A quick recount on the actual structure indeed shows eight -OH groups. Option C therefore matches the accepted structure and is true.

Testing option D. The glycosidic bond in lactose is cleaved by dilute acid or by the enzyme lactase. One then simply obtains the two original monosaccharides in the same configuration:

$$\text{lactose}+ \text{H}_2\text{O}\;\xrightarrow[\text{H}^+]{\text{hydrolysis}}\; \text{D(+)-glucose}+ \text{D(+)-galactose}.$$

Hence option D is also true.

Only option A contradicts the real $$\beta(1\!\rightarrow\!4)$$ linkage of lactose, while the other three statements agree with accepted carbohydrate chemistry.

Hence, the correct answer is Option A.