Long chains formed by the process of polymerization are called polymers, which are typically made up of smaller units called monomers. So, the correct answer from your options would be monomers.
To clarify:
- Subunits is a general term and can refer to smaller components in a larger structure but is not specific to polymers.
- Ethylenes are a type of hydrocarbon and can be used as monomers in polymerization, but they do not represent the chains themselves.
- DNA strands are polymers made up of nucleotide monomers, but in the context of the original question, "monomers" is the most accurate term.
Therefore, "monomers" are the subunits that are polymerized to form long chains.