What characterizes a polymer?

A polymer is characterized by being a large molecule composed of repeating structural units, which are often referred to as monomers. These monomers are covalently bonded in a process known as polymerization, resulting in a macromolecule that can vary widely in size and complexity. This structural characteristic is what distinguishes polymers from smaller, simpler molecules.

Polymers can be found in many forms, including natural materials like proteins and nucleic acids, as well as synthetic substances like plastics and fibers. Their properties depend significantly on the nature of the repeating units and the way they are linked together.

The other choices do not capture the essence of what defines a polymer. A simple molecule with a single unit describes monomeric substances rather than polymers. The dissolution of a molecule in water relates to its solubility characteristics and does not define whether it is a polymer. Lastly, the classification of a molecule as gaseous refers to its physical state, which does not correlate with the structural complexity required of polymers.