Final answer:
Glucose is the most common monosaccharide subunit in food polysaccharides, as it is abundant in nature and used extensively by living organisms for energy.
Step-by-step explanation:
The most common monosaccharide subunit in food polysaccharides is glucose. This is because glucose is the most abundant monosaccharide in nature, as it is produced by plants through photosynthesis and is used by the cells of most organisms for energy. Although galactose and fructose are also common monosaccharides and share the same chemical formula with glucose (C6H12O6), they are structurally and chemically different because of the arrangement of functional groups around their asymmetric carbons, making them isomers of glucose. Glucose is an aldohexose, while fructose, found in fruit and in sucrose, is a ketohexose. Sucrose itself is a disaccharide comprised of glucose and fructose.