Which monosaccharide rarely occurs freely in nature but is tied up in milk sugar until it is freed during digestion?

Galactose

Explanation:

Milk sugar contains the disaccharide called lactose.

A disaccharide is formed from the condensation of two monosaccharides.

Lactose is formed from the condensation of glucose and galactose.

Galactose rarely occurs freely in nature. It is freed when the enzyme lactase breaks down lactose during digestion.