As you can see in the diagram presented below, the correct genotypes of the parents are TT and Tt.
Each parent give its children only one chromosome of the chromosome pair, therefore only one allele of the gene.
In our case, TT and Tt is the correct answer because one of the parents has to have the t allele to give to the Tt offspring.
But if both of the parents were Tt, one of the offspring would have tt genotype, and if the parents were TT and tt, all of the offspring would be Tt. Therefore, the TT x Tt combination is the only possible answer.
