"59. maize (corn) plants are tall if they have the genotype hh or hh, and they are short if they have genotype hh. if a tall plant is mated with a short plant, which outcome would indicate that the tall parent corn plant was heterozygous"

[Assuming the tall maize plant's genotype is either Hh or HH, and the short plant's genotype is hh]

The outcome is that a short plant of genotype hh will occur.

WHY IS THIS SO?
If the tall plant is homozygous, it's genotype will be HH. When mated with a short plant of genotype hh, the results will be 100 percent chance of short off springs which are heterozygous, with genotype Hh.

However, if a heterozygous plant of genotype Hh, was used to mate with the short plant of genotype hh, there will be a 50 percent chance of getting a offspring with genotype hh and phenotype of being a short maize plant.

METHOD TO DETERMINE OFFSPRING'S GENOTYPE IS IN THE PICTURE ABOVE.

Your answer here is d