Final answer:
In Mendel's experiments, 75% or three-fourths of the F2 generation plants were tall, reflecting a 3:1 phenotypic ratio of dominant to recessive traits.
Step-by-step explanation:
In Mendel's experiments, when Mendel allowed the F1 generation of tall pea plants to self-pollinate, the resulting F2 generation displayed a remarkable pattern. What Mendel observed was that 75% of the F2 plants were tall. This outcome is described as a 3:1 phenotypic ratio, where three out of every four plants exhibited the dominant phenotype, in this case, tallness. Therefore, the proportion of F2 plants that were tall is three-fourths (option b). Mendel's pioneering work laid the foundation for our understanding of inheritance and genetics.