Glucose is a vital molecule in the field of biochemistry and is often referred to as the primary source of energy for living organisms. It belongs to the category of carbohydrates, specifically a monosaccharide.

Monosaccharides are the simplest form of carbohydrates, also known as simple sugars. They are composed of a single sugar unit and cannot be further hydrolyzed to yield simpler sugars. Glucose, with its chemical formula C6H12O6, is a hexose, which means it contains six carbon atoms.

Now let's analyze each option given and determine why glucose cannot be classified as the mentioned options:

a) A Hexose:
- Glucose is indeed a hexose since it contains six carbon atoms. Therefore, it can be classified as a hexose.

b) A Carbohydrate:
- Carbohydrates are organic compounds made up of carbon, hydrogen, and oxygen atoms. Glucose, being composed of these elements, is considered a carbohydrate. Therefore, glucose can be classified as a carbohydrate.

c) An Oligosaccharide:
- Oligosaccharides are carbohydrates composed of a few (2-10) monosaccharide units linked together. Since glucose is a monosaccharide, it cannot be classified as an oligosaccharide.

d) An Aldose:
- Aldoses are a type of monosaccharide that contain an aldehyde functional group (-CHO). Glucose, on the other hand, contains a carbonyl group in the form of a ketone (-C=O). Therefore, glucose cannot be classified as an aldose.

In conclusion, glucose cannot be classified as an oligosaccharide because it is a monosaccharide. It is, however, a hexose and a carbohydrate due to its chemical composition. Additionally, it cannot be considered an aldose since it does not contain an aldehyde functional group.