A good description of an allele

A good description of an allele. Which of the following is a good description of an allele? A. It is a version of a gene produced by natural selection. B. It is a more modern and improved version of a particular gene. C. One of only two versions of a gene that we see within populations. D. It is one version of a particular gene. E. It is a dysfunctional version of a particular gene.