Macromolecules are the giant molecules formed by joining a number of smaller similar or two or more types of molecules. In the process of polymerization, smaller molecules are joined by the formation of chemical bonds and thus form large complex molecules called polymers. Therefore, option (a) is incorrect.
Carbohydrates, proteins, lipids and nucleic acids are examples of complex organic compounds found in living organisms. They are macromolecules. The polysaccharides in carbohydrates are formed by the joining of many monosaccharide units. Therefore, option (b) is incorrect.
Monomer is a unit of the polymer. Many monomers join together to form a complex molecule by the process of polymerization. Therefore, option (d) is correct.
Polymer is the product formed by the polymerization of many monomers. Therefore, option (c) is incorrect.
Hence, in polymerization, complex molecules are formed by the joining together of many (d) monomers.