Answer:
B. They consist of different sequences of DNA
Explanation:
Alleles are contrasting or alternative forms of a gene. A diploid organism i.e an organism with two sets of homologous chromosome, contains two alleles for each gene that encodes a trait. Although the two alleles of that particular gene encodes the same trait, they can encode different phenotypes i.e way it is expressed because they contain different sequences of DNA.
For example, in a gene coding for height in pea plants, two alleles, T and t are responsible. Allele T codes for tall height (phenotype) while allele t codes for short height (phenotype). Although the two alleles are for the same trait, they differ in the content of their DNA sequence, hence, are expressed differently or variably.