Individuals who lack lactase are unable to break down the sugar lactose. which term best describes lactase?

The term is enzymes. The lactase enzyme is produced by many organisms and is used for the breakdown of lactose, in order to utilize whole milk. People who lack lactase, are not able to utilize and become lactose-intolerant.
In humans, Lactase is present in the brsuh border of the small intestines. It is a beta-galactosidase enzyme and hydrolysis the disaccharide lactose into glucose and galactose monomers. Commercially, it is isolated from the yeasts. to cure lactose-intolerance.