Answer: The correct answer for the blank is- C.) variations in inherited characteristics.
Alleles cane be described as the variant/ alternative form of a genes that corresponds to specific phenotypic trait ( morphological for of an organism).
For a particular gene, primarily there can be two variants that is dominant and recessive allele. They are responsible for variations in inherited characteristic ( for instance, two copies of recessive allele will exhibit a different trait in the individual as compared to inheritance of a dominant allele).
Thus, option C) is the right answer.