Final answer:
Alleles are different versions of the same gene, varying in their sequence of nucleotides, and contribute to genetic diversity by resulting in different traits. They can be homozygous or heterozygous in an individual's genotype. Option C is correct.
Step-by-step explanation:
Alleles are best defined as versions of the same gene that differ by their sequence of nucleotides. Alleles are responsible for the variation of traits within a species, as they represent different versions of a gene that may lead to different traits.
For example, the human chromosome responsible for blood type can have sequences A, B, or O, and these sequences are different alleles of that gene. Individuals may inherit the same allele from both parents, making them homozygous (like AA or OO), or they can inherit different alleles, making them heterozygous for the trait (like AB or AO).
The combination of alleles an individual inherits contributes to their genotype, and the physical expression of these alleles is their phenotype.