Answer:
The correct answer would be alleles.
Alleles can be defined as the alternative forms of the same gene.
They are present at same loci in homologous chromosomes.
If both the alleles of the gene are same, then the organism is said to homozygous with respect to that particular gene. For example, TT, tt, YY, yy et cetera.
If homologous chromosomes consist of different alleles of the gene, then the organism is said to be heterozygous for that gene. For example, Tt, Yy, et cetera.