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Carbohydrates are a class of macromolecules that are a major source of energy to the cell and provide structural support to plants, fungi, and arthropods. Carbohydrates are of three types: monosaccharides, disaccharides, and polysaccharides. Monosaccharide units join together to form disaccharides or polysaccharides.

Disaccharides are formed by glycosidic bonds between two monosaccharide units. As lactose is already a disaccharide, it cannot link with glucose to form a disaccharide. Hence, option a is incorrect.

Disaccharides are formed by a glycosidic linkage between two monosaccharide units. Sucrose is a disaccharide produced by the combination of glucose and fructose. Hence, option c is incorrect.

Monosaccharide units join together by glycosidic bonds in a dehydration reaction to form disaccharides. Fructose is a monosaccharide that combines with glucose to form sucrose. Hence, option d is incorrect.

When glucose and galactose monomers combine in a dehydration reaction they form lactose. Glucose and galactose are linked to one another by glycosidic linkage. Hence, option b is correct.