To solve this problem, we need to understand the concept of alleles. Let's analyze each option:$\newline \bullet$Option 1: Different phenotype.$\newline$ - Phenotype refers to the observable characteristics of an organism, which can be influenced by alleles, but alleles themselves are not phenotypes.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 2: True breeding homozygotes.$\newline$ - True breeding refers to organisms that produce offspring of the same phenotype when self-fertilized. Homozygotes have identical alleles for a trait, but alleles themselves are not defined as true breeding homozygotes.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 3: Different molecular forms of a gene.$\newline$ - Alleles are indeed different versions or forms of a gene that arise by mutation and are found at the same place on a chromosome.$\newline$ - This is the correct definition of alleles.$\newline \bullet$Option 4: Heterozygotes.$\newline$ - Heterozygotes have two different alleles for a particular gene, but alleles themselves are not heterozygotes.$\newline$ - Therefore, this option is incorrect.$\newline$Based on the analysis, the correct option is Option 3: Different molecular forms of a gene.