Question

An organic compound contains only the elements carbon, hydrogen, and oxygen. Could this compound be a carbohydrate? Could it be a protein?

Answer 1

Yes, it could be a carbohydrate
No, it can not be a protein.

Explanation:

A carbohydrate has only Carbon, Hydrogen and Oxygen, with the hydrogen and oxygen in the ratio 2:1.
Examples are glucose `C_6H_12O_6` and table sugar `C_12H_22O_11`

A protein is made of amino acids. Amino acids have an amino group which is `-NH_2`.
So without nitrogen atoms the compound can not be a protein.