Which diagram shows a homologous chromosome pair that has heterozygous alleles?

The right answer is C (see the images):

An organism is heterozygous for a gene when it has two different alleles of that gene on the same locus for each of its homologous chromosomes.
An allele is a variable version of the same gene or genetic locus, that is to say, a varied form that can be distinguished by variations in its nucleotide sequence.

The littoral zone. . .