D is the best of the options given. Alleles are different versions of a gene. A given gene is localized (located) in a specific location on a chromosome. A chromosome consists of genes. There are many different versions of genes out there. (i.e. the gene for hair color could have an allele coding for black, blond, red, brown, etc. colored hair. That allele would be where you find the gene for hair color on the chromosome.) Hope this helps! :)