What is a heritable factor consisting of a length of DNA that codes for a protein called?

A heritable factor consisting of a length of DNA that codes for a protein is called a gene. Genes are fundamental units of heredity and are responsible for the transmission of traits from parents to offspring. Each gene contains the specific instructions necessary for producing particular proteins, which ultimately contribute to an organism's phenotype and functions.

Chromosomes are structures within cells that organize and store genetic material, but they contain many genes rather than being a single factor that codes for a protein. Alleles refer to different versions of a gene that can exist at the same locus on a chromosome, influencing variations in traits. Traits themselves are the observable characteristics of an organism that result from the expression of genes and their interaction with the environment. Thus, the definition points directly to a gene as the correct answer.