Which of the following best describes a monomer?

A monomer is best described as a small molecular unit that can combine to form larger molecules, known as polymers. This definition emphasizes the building-block nature of monomers in various biochemical processes. For instance, in the case of proteins, amino acids serve as monomers that link together through peptide bonds to create polypeptides and ultimately functional proteins. Similarly, in carbohydrates, simple sugars like glucose act as monomers that can polymerize to form starch or cellulose.

The other options refer to concepts that are distinctly different from the definition of a monomer. For instance, an allele that is always expressed if present pertains to genetics and the dominance of traits rather than molecular building blocks. A technique for visualizing molecular structures is related to methods used in biochemistry and molecular biology, such as X-ray crystallography or NMR spectroscopy, and does not describe what a monomer is. Lastly, a genetic variation that is recessive discusses allele expression in hereditary traits, which again is unrelated to the concept of monomers in the context of molecular biology or chemistry. Thus, the selection accurately captures the essence of what a monomer is and its role in the formation of larger molecular structures.