Final answer:
Segregation refers to the separation of alleles during gamete formation, which occurs during meiosis, ensuring that each gamete carries a single allele for each gene.
Step-by-step explanation:
The word 'segregation' in the context of T and t alleles refers to the separation of alleles during gamete formation. Segregation occurs during the process of meiosis, where each gamete receives only one allele for each gene. Therefore, the correct answer to what segregation means with respect to alleles is D) Separation of alleles during gamete formation. This principle ensures that offspring inherit one allele for each gene from each parent, resulting in the genetic diversity seen in sexual reproduction.