Hint: Enzymes are the class of biomolecules that catalyze biochemical reactions. The enzymes act upon the substrate molecules and convert them into products.
Option A: Proteins are products of ribosomal translation and they are composed of sequences of amino acids which are also known as polypeptide chains. Protein composes the majority of organic molecules and is highly diverse. Enzymes, hormones, structural elements, etc. are examples of proteins. Therefore this is the correct answer.
Enzymes are globular proteins or folded proteins. In enzymes, protein is folded in such a way that it can bind with substrates specifically. The binding site for different enzymes differs according to the specific substrate with which it binds.
Option B: Nucleic acid is the polymers of nucleotides which are composed of a five-carbon sugar, phosphate group, and nitrogen bases. RNA and DNA are nucleic acids and they are the most important biomolecules because they encode and store all the genetic information of all cells and organisms. Therefore, this is the incorrect option.
Option C: Carbohydrates are saccharides or chains of saccharides, which are composed of carbon, hydrogen, and oxygen molecules. They are macronutrients and are the main source of energy for body functions. They are obtained from fruits, grains, etc. Starch, sugar, cellulose, etc. are examples of carbohydrates. Therefore, this is the incorrect option.
Option D: Lipids are a class of biomolecules that are synthesized from fats. They are soluble in nonpolar solvents. Lipids are structural components of cell membranes and are responsible for the storage of energy also. Some hormones are also lipids. Eg: phospholipid, sterols. Therefore, this is the incorrect option.
Hence the correct answer is option ‘A’ i.e, Protein.
Note: The activity of enzymes is completely dependent on their structure because this determines the structure of their active site and the substrates to which it binds and the reaction it catalyzes. They are active only at specific ranges of temperature and pH conditions, outside which the protein denatures and the structure collapses.