Which of the following is not a chemically modified sugar?
1. Glucosamine
2. N-acetyl galactosamine
3. Galacturonic acid
4. Dihydroxy acetone

Dear student, 
The correct option is (4) Dihydroxyacetone. It is a simple saccharide with 3 carbon atoms. It is commonly called glycerone. 
Glucosamine and N-acetyl galactosamine are amino sugars. Galacturonic acid is formed by oxidation od D-galactose. It is a sugar acid. 
Regards!