Multiple alleles are present
A. At the same locus of the chromosome
B. On non-sister chromatids
C. On different chromatids
D. At different loci on the same chromosome

Hint: Multiple alleles is the presence of more than two alternate forms of a gene. They are accommodated on two loci on a homologous pair. This is stimulated by the presence of more than one allele on the same locus of a chromosome. This means one allele is present at a locus at a time.

Complete answer:
An allele is a variant form of a gene. Each gene resides at a specific locus (location on a chromosome) in two copies, and one copy of the gene is inherited from each parent. These copies are not necessarily the same. Genes have a variety of different forms, located at the same position, or genetic locus, on a chromosome. Humans are called diploid organisms as they have two alleles at each genetic locus.

> Option A- at the same locus of the chromosome
Alleles occupy a specific spot called locus on a chromosome. In diploid organisms, only two alleles are present for a gene. Example- Humans. Multiple allelism is the condition in which a gene exists in more than two allelic forms.

> Option B- On non-sister chromatids
This is incorrect as alleles are present on a specific spot called locus on the same chromosome.

> Option C- On different chromatids
This is also incorrect. Alleles are not present on different chromatids.

> Option D- At different loci on the same chromosome
The alleles are present on a specific spot on a chromosome known as locus or loci. They are not present in different loci.

Hence, the correct answer is option (A).

Note: Example of multiple-allele genes is two humans of the ABO blood group system and the human-leukocyte-associated antigen (HLA) genes. The ABO system in humans is regulated by three alleles, usually referred to as IA, IB, and IO.