A heterogenous plant can be described as a heterozygous plant in which the alleles of a gene are different. For example, in the above question, the pea plant has one allele which is T and the other one which is t.
According to Mendel's law of segregation, the alleles assort independently at the time of gamete formation. Hence, there are equal chances that a gamete will carry T or t gene. Hence, the pollen grain will carry any one of the two alleles for the height gene.
