Answer:
The tall parent was heterozygous
Step-by-step explanation:
If tall height is dominant to short height the only genotype possible for short height is hh, while there are two possible genotypes for tall height, Hh and HH. When HH is crossed with hh all the offspring are Hh, but if Hh is crossed with hh, a quarter of the offspring is HH, a quarter is hh and half is Hh. If the tall pea plants were HH there would only be tall offspring, but because there are some short offspring we know that the tall pea plants must have a genotype of Hh.