The correct answer to the question is option 3: allele. An allele is a variant form of a gene that controls a specific trait. In genetics, individuals inherit two alleles for each trait, one from each parent. When these alleles are the same, they are called homozygous, and when they are different, they are called heterozygous. This is where the concept of contrasting characters comes into play. A pair of contrasting characters controlling the same trait refers to two different alleles for that specific trait. These alleles may be responsible for different variations or expressions of that trait. It is important to note that factors (option 1), loci (option 2), and lineage (option 4) are not accurate terms for describing a pair of contrasting characters controlling the same trait.