Hi!
The answer would be: Dominant
A cross between a true breeding short plant (homozygous for short, suppose tt), and a cross between a true breeding tall plant (homozygous for tallness, suppose TT) would result in all the offspring being heterozygous for the traits, meaning they would each have an allele of shortness (t) and an allele of tallness (T).
Considering the offspring were all tall, we understand that the allele for tallness must have been dominant over the allele for shortness to present itself over it. Hence, the term describing the gene for tallness would be dominant, and the term describing the gene for shortness would be recessive.
Hope this helps!
