Concept:An allele is one of the different versions of a gene.
Explanation:A gene can exist in more than one alternate form.
Each alternate form is known as an allele.
Alleles occupy the same position, called a locus, on homologous chromosomes.
Different alleles may produce different traits, such as red or white flower colour in a plant.
For example, one allele may code for red flowers, while another allele codes for white flowers.
Thus, the "alternate form of a gene" is best described as an allele.
Answer:D. Allele