Identify the polymers that are found in natural sources. not all of the polymers listed are found in natural sources.

From the polymers listed, the ones which are found in natural sources are: protein, cotton and silk.
A polymer is a large molecule composed of many repeated subunits. Many small molecules (monomers) are bound by chemical reactions, called polymerization, and form the polymers. Polymers are widely used in many applications and they can either be synthetic or naturally occurring biopolymers. In the provided list, polypropylene, polyvinyl chloride and nylon are synthetic polymers. Polypropylene and nylon are thermoplastic polymers, and polyvinyl chloride (vinyl) is a plastic polymer. On the other hand, proteins are naturally occurring polymers produced by many amino acids. Cotton is formed by repeating units of cellobiose and it is a naturally occurring cellulose polymer. Finally, silk is a polypeptide and a linear fibroin polymer.