Answer:
TT
All tall
Explanation:
If an organism is purebred, that means it is homozygous. That means, it contains two copies of the same allele (trait) at this particular gene. Lets denote the tall allele as T. That means the plant is TT, and purebred tall.
No matter what genotype (i.e. what 2 alleles) another plant has, the offspring will always be tall. That is because it will always inherit one T from the TT parent. Even if we cross it to a tt plant, all the offspring would be Tt. They would be heterozygous, but they would be tall.
