The polyhydroxy aldehydes are called:
(a)- Proteins
(b)- Fats
(c)- Carbohydrates
(d)- Vitamins

Hint: Some of the polyhydroxy aldehydes or polyhydroxy ketones are sweet. The polyhydroxy aldehydes or polyhydroxy ketones are very essential for our body. Our diet has mostly these molecules.


Complete step by step answer:
We define polyhydroxy aldehydes or polyhydroxy ketones are those carbon compounds, in which the number of carbon atoms is high, where more than one carbon atom has an alcohol or hydroxyl group ($-OH$) group and the terminal carbon has either the aldehyde ($-CHO$) or the ketone ($-C=O$) group.
These compounds are known as Carbohydrates.
The older definition of carbohydrates says that carbohydrates are those which are only composed of carbon, hydrogen, and oxygen; the hydrogen and oxygen are present in the same ratio of water.
But the modern definition of carbohydrates says that the compounds which are polyhydroxy aldehydes or polyhydroxy ketones that are optically active or substances that on hydrolysis gives these compounds.
Carbohydrates are of three types which are namely: Monosaccharides, Disaccharides, and polysaccharides.
Monosaccharides are the simplest carbohydrates like glucose, erythrose, mannose, galactose, etc.
Disaccharides are the compounds that give two monosaccharides on hydrolysis like sucrose, maltose, lactose, etc.
Polysaccharides are the compounds that give a huge number of carbohydrates on hydrolysis like starch, cellulose, etc.

So the correct answer is an option (c)- Carbohydrates.


Note: Carbohydrates can also be classified as reducing or non-reducing sugars. Reducing sugars have a free aldehyde or ketone group that can give Tollen’s test. Non-reducing sugar doesn't have a free aldehyde or ketone group and doesn't give Tollen’s test.