What is the basic unit that can join together with other small units to form polymers?

The correct answer is the monomer. Monomers are the simplest form of molecules that can bond together to create larger structures known as polymers. Each monomer consists of a single, small molecule that retains the characteristic properties of that type of compound. When multiple monomers undergo a process called polymerization, they link up to form chains or networks, resulting in a polymer. This process is fundamental in the formation of various biological molecules, such as proteins (formed from amino acid monomers) and nucleic acids (formed from nucleotide monomers).

In contrast, a polymer is already a large structure made up of many monomers, and macromolecules typically refer to large molecules that can include polymers but also have other structures and complexities. Polyatomic ions, on the other hand, refer to charged species made up of two or more atoms, which is unrelated to the concept of building polymers from monomers.