The right answer is C
An allele is a variable version of the same gene, because a gene can have several versions of the same gene, which can give slightly different structural proteins due to these variations in the DNA, thus, giving different phenotypes.
In most cases, the multiplicity of alleles in a gene gives phenotypic variation in the population due to their different expression. Therefore, the larger the number of alleles, the greater the number of phenotypes in the population.
To a lesser extent (more rarely), some variation can be a deletion giving non-functional proteins, causing rare diseases (like cystic fibrosis, hemophilia, sickle cell anemia).
