Final answer:
Tagatose is a ketohexose, a type of carbohydrate with a ketone group in the middle of its six-carbon chain and considered to be a D-sugar due its hydroxyl group orientation on the fifth carbon.
Step-by-step explanation:
The carbohydrate tagatose can be classified by both the type of carbonyl group present and the number of carbon atoms in the molecule. Since tagatose has a carbonyl group in the middle of the carbon chain, it is classified as a ketose. Furthermore, because tagatose has six carbon atoms, it is called a hexose.
In the Fischer projection of D-tagatose, the presence of the carbonyl group linked to two other carbon atoms indicates that it is a ketonic sugar, hence a ketose. Additionally, since the molecule is a six-carbon sugar, it can further be categorized as a hexose. The D- configuration designation comes from the orientation of the hydroxyl group on the fifth carbon, and in the case of D-tagatose, this group is positioned on the right, confirming its D-configuration.