Final answer:
Disaccharides are formed through a dehydration reaction between two monosaccharides, resulting in a glycosidic bond connecting them.
Step-by-step explanation:
Disaccharides are made from the dehydration reaction of two monosaccharides. During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. This bond is specifically known as a glycosidic bond, and it can take on alpha or beta forms depending on the orientation of groups around the ring structure. The formation of a disaccharide, therefore, is not a result of substitution, reduction-elimination, or oxidation-reduction reactions.