The answer is D. a gene that causes a specific trait
An allele is specific variation of a gene. Bacteria, because they have a single ring of DNA, have one allele per gene per organism. In sexually reproducing organisms, each parent gives an allele for each gene, giving the offspring two alleles per gene. Because alleles are just variants of specific genes, different alleles are found on the same locations on the chromosomes of different individuals. This is important because it gives organisms to be incredibly varied in the functions of their various alleles, while at the same time being able to reproduce. This create variety caused by mutations in specific genes gives rise to a wide number of alleles for any trait in a given population.