What is the term for a length of DNA coding for a particular protein?

The correct term for a length of DNA that codes for a particular protein is "gene." Genes are segments of DNA located on chromosomes that contain the instructions for making proteins, which perform various functions in the body. Each gene is a specific sequence of nucleotides that is transcribed into mRNA and then translated into a protein. This fundamental unit of heredity plays a crucial role in determining an organism's traits and functions.

In the context of the other choices, "allele" refers to different versions of the same gene, "chromatid" describes one half of a duplicated chromosome, and "codon" is a sequence of three nucleotides in mRNA that corresponds to a specific amino acid during the protein synthesis process. While all these terms relate to genetics and molecular biology, only "gene" specifically signifies the stretch of DNA that encodes for a protein.