To determine whether a tall plant is heterozygous or homozygous what should it be crossed with

To determine whether the plant is heterozygous or homozygous, it should be crossed with a heterozygous plant.

This is because, let's say that the plant in question is heterozygous. When crossed with this known heterozygous plant, some of the offspring will then be homozygous recessive, and thus display the recessive phenotype (short plant) for the gene being tested.

If the plant in question is homozygous, all of the offspring will display a dominant phenotype (tall plant) due to the masking of the recessive gene of the known heterozygous plant.

I hope I explained it clearly enough! If you have any questions, you can comment in the comment box below.