The right answer is: C. One form of a gene.
The human body consists of 23 pairs of chromosomes. Each chromosome contains many information in the form of genes that express the hereditary characteristics of an individual.
Each gene has two alleles, one from the genetic information of the father, the other from that of the mother. If the two alleles of the individual are identical, the characteristic resulting from this allele will be expressed. If, on the other hand, they are different, the so-called dominant allele will be imposed on the other recessive allele, and it is this dominant allele which will give its characteristic to the individual.
By way of example, the color of the eyes: if a child inherits a "brown" allele and a "blue" allele, he will have brown eyes, since the brown allele is dominant in relation to the allele blue that is recessive.