Which of the following best describes an allele?

An allele is best described as a variant form of a gene. Genes are segments of DNA that carry the instructions for the development and functioning of an organism, and alleles are different versions of these genes that may produce different traits. For example, a gene for flower color in a plant may have several alleles, such as one for red flowers and another for white flowers. The presence of different alleles contributes to genetic diversity within a population, influencing traits and characteristics in offspring.

The other options describe related concepts but do not accurately define an allele. A complete set of genes refers to the organism's genome, while a section of DNA that codes for proteins describes a gene itself. A type of chromosome indicates a structural component of the cell, not specifically the variations of genes. Therefore, identifying an allele as a variant form of a gene captures its role in genetics clearly and accurately.