D) The offspring can be tall or short.
It's helpful to use a Punnet Square in this situation. We know that the F1 plants are all Tt (heterozygous) because they have to get a T from the TT parent and a t from the tt parent. Now we cross the Tt plants:
T | t |
T | TT | Tt |
t | Tt | tt |
So, there will be some plants that are tall (TT or Tt) and some plants that are short (tt). Hope this helps! :)