Explanation:

Hexose sugars are a type of monosaccharide that contain six carbon atoms. They are commonly found in various biological systems and play important roles as energy sources and structural components. The options provided in the question are various hexose sugars, and we need to determine which one is NOT a hexose sugar.

Hexose Sugars:
Hexose sugars are characterized by having six carbon atoms in their structure. They can exist in either an open-chain form or a cyclic form, with the cyclic form being more common in physiological conditions. Some examples of hexose sugars include glucose, fructose, galactose, mannose, and ribose (a pentose sugar).

Option B: Xylose
Xylose is a five-carbon sugar, also known as a pentose sugar, rather than a hexose sugar. It is a component of hemicellulose, a complex carbohydrate found in plant cell walls. Xylose is not a hexose sugar because it does not have six carbon atoms in its structure. Therefore, option B (Xylose) is the correct answer.

Other Options:
- Option A: Idose: Idose is a rare sugar that is derived from glucose. It is an epimer of glucose, meaning it has a different configuration at one carbon atom. Idose is a hexose sugar because it contains six carbon atoms in its structure.
- Option C: Gulose: Gulose is an artificial sugar that is not commonly found in nature. It is a hexose sugar because it contains six carbon atoms in its structure.
- Option D: Altrose: Altrose is an aldohexose sugar that is a stereoisomer of glucose. It is a hexose sugar because it contains six carbon atoms in its structure.

In conclusion, Xylose is the correct answer because it is not a hexose sugar. The other options (Idose, Gulose, and Altrose) are all hexose sugars because they contain six carbon atoms in their structure.