Which class of compounds contains only carbon, hydrogen, and nitrogen?

Nitrogen forms many thousands of organic compounds. Most of the known varieties may be regarded as derived from ammonia, hydrogen cyanide, cyanogen, and nitrous or nitric acid. The amines, amino acids, and amides, for example, are derived from or closely related to ammonia.

A carbohydrate has only Carbon, Hydrogen and Oxygen, with the hydrogen and oxygen in the ratio 2:1.

Examples are glucose C6H12O6 and table sugar C12H22O11

A protein is made of amino acids. Amino acids have an amino group which is −NH2.  

So without nitrogen atoms the compound cannot be a protein.

Nucleic acids are organic compounds that contain carbon, hydrogen, nitrogen but they also contains phosphorus and oxygen. They are made of smaller units called nucleotides.

So;

Simple Amines are the class of organic compounds that only contains Carbon, Hydrogen and Nitrogen.

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.