Answer: Option (a) is the correct answer.
Explanation:
When two monosaccharide units chemically combine together through a glycosidic linkage then it leads to the formation of a double sugar which is known as a disaccharide.
For example, sucrose, lactose etc are disaccharides.
Monosaccharides are the basic building blocks of carbohydrates and they are also known as simple sugars. These are also water soluble in nature.
For example, glucose is a monosaccharide.
Polysaccharides are defined as the substances which contain number of monomer units. For example, starch, cellulose, glycogen etc are polysaccharides.
Thus, we can conclude that a disaccharide is formed by the chemical bonding of two monosaccharides.