Well the parental genotypes in this case, are that they are both homozygous recessive, as they possess the recessive allele thus showcasing the recessive phenotype assuming that the mode of inheritance is complete dominance. Since each parent can only donate t allele to their offspring and no other, the probability that the offspring will also be tt is the product of the probable alleles that the parents can give.
It is 100% or 1.00.