which is a basic unit of a sugar molecule

Basic unit of sugar molecule:   Monosaccharides  also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. (though not all molecules with this formula are monosaccharides). 

They are usually colourless, water soluble and crystalline solids. Contrary to their name (sugars), only some monosaccharides have a sweet taste.  Most monosaccharides have the formula CnH²nOn (though not all molecules with this formula are monosaccharides).

Examples of monosaccharides include glucose (dextrose), fructose (levulose), and galactose .  Monosaccharides are the building blocks of dissachride  (such as sucrose and lactose) and polysaccharides (such as cellulose and starch). The table sugar used in everyday vernacular is itself a disaccharide sucrose comprising one molecule of each of the two monosaccharides D-glucose and D-fructose.