Which of the following describes a simple compound whose molecules can join together to form polymers?

A monomer is a simple compound whose molecules can join together through covalent bonds to form larger structures called polymers. This process of joining monomers together is often referred to as polymerization. Monomers are the fundamental building blocks of polymers, which can be natural, like proteins and nucleic acids, or synthetic, like plastics.

Polymers themselves are not simple compounds; they are made up of many repeating units of monomers. A compound is a broader term that refers to any substance formed from two or more different elements that are chemically bonded together. A reagent is a substance that causes a chemical reaction but does not specifically refer to the building blocks of larger molecules. Thus, in the context of polymer formation, the term that accurately describes a simple compound that combines to create polymers is indeed monomer.