What are the different forms of the same gene that can result in variations…

Concept: An allele is one of two or more alternative forms that the same gene can take at its fixed position (locus) on a chromosome. Because different alleles of a gene can direct slightly different versions of that gene's product, they are the source of inherited variation between individuals — such as differences in eye colour or disease risk.

Application: The question describes "different forms of the same gene" that "result in variations such as eye colour or disease risk" — this is exactly the definition of an allele, so the keyed option, Alleles, is correct.

Cross-check — contrasting each option by what it actually is:

• Genomes: the complete set of an organism's genetic material — every gene together with all non-coding DNA — describing the whole genetic content, not one alternative version of a single gene.

• Chromosomes: a single coiled structure of DNA wound around proteins that carries many genes strung along its length — the physical carrier of genes, not a version of one gene.

• Nucleotides: the basic chemical subunit of DNA/RNA (a sugar, a phosphate group, and a nitrogenous base) that strands are built from — a molecular building block, not a form of a gene.

• Alleles: alternative forms of the same gene at the same locus — this matches the stem's description precisely.

Result: only "alleles" names alternative forms of one gene, confirming the keyed answer.