Understand the definition of a monosaccharide: Monosaccharides are the simplest form of carbohydrates and are single sugar molecules. They cannot be hydrolyzed into simpler sugars.
Identify the given options: Sucrose, Glucose, Fructose, and Galactose.
Recall that Glucose, Fructose, and Galactose are all monosaccharides. They are simple sugars with the general formula \( C_6H_{12}O_6 \).
Recognize that Sucrose is a disaccharide, not a monosaccharide. It is composed of one glucose molecule and one fructose molecule linked together.
Conclude that the option which is not a monosaccharide is Sucrose, as it is a disaccharide.