Lactose forms via β-1,4-glycosidic bond between β-glucose and β-galactose

Steps:

• Lactose is a disaccharide made from one glucose and one galactose unit.
• The glycosidic linkage is β-1,4, where C1 of galactose bonds to C4 of glucose.
• Galactose participates in β-anomeric configuration for the bond.
• Glucose remains in β-pyranose form, with its anomeric carbon as the reducing end.

Why D is correct:

• Lactose structure is β-D-galactopyranosyl-(1→4)-β-D-glucopyranose, confirming β-anomers of both sugars condense.

Why the others are wrong:

• A: α-galactose would form α-1,4 linkage, not matching lactose's β bond.
• B: α-glucose alters the reducing end configuration, inconsistent with standard β-glucose in lactose.
• C: β-glucose is correct, but α-galactose creates wrong anomeric linkage.

Final answer: D