The correct answer is option-3, nitrogen. In a pure sugar, the elements carbon, hydrogen, and oxygen are always present. Sugars are organic compounds made up of carbon, hydrogen, and oxygen atoms. These three elements are essential for creating the chemical structure of sugars.

Carbon is the main element found in sugars, forming the backbone of their molecular structure. Hydrogen atoms are attached to the carbon atoms and provide stability to the molecule. Oxygen atoms are also present in sugars and are typically found in functional groups such as hydroxyl groups (-OH) and carbonyl groups (C=O).

On the other hand, nitrogen is not typically found in sugars. Nitrogen is more commonly found in compounds such as amino acids, proteins, and nucleic acids.

Therefore, among the given options, nitrogen is not present in a pure sugar. The other options - carbon, hydrogen, and oxygen - are all essential components of sugars.