Answer: A. All were tall.
Step-by-step explanation:
True-breeding plants are plants with completely dominant or recessive genotypes (TT (tall) and tt (short), for this example). He crossed one tall plant with genotype TT with a short plant with genotype tt. Set up a Punnett square with parents TT and tt.
All offspring will have the genotype Tt. T is dominant, so only one copy of it is needed for a plant to be tall. All of the offspring have one copy of T, so they're all tall.