Final answer:
The plants that must all be heterozygous are the purple-flowered plants in the F1 generation (answer B). This is because they are the result of a cross between homozygous dominant purple-flowered and homozygous recessive white-flowered plants in the P generation.
Step-by-step explanation:
The question refers to Gregor Mendel's experiments on inheritance patterns in pea plants. When Mendel crossed pure-breeding purple-flowered plants with pure-breeding white-flowered plants in the P (parental) generation, all the resulting F1 generation offspring had purple flowers, indicating that purple flower color is a dominant trait over white flower color.
In the F2 generation, which resulted from the self-pollination of F1 plants, there was a 3:1 ratio of purple to white flowers. This means that the only plants that must be heterozygous are the purple-flowered plants in the F1 generation (answer B), as they all displayed the dominant purple phenotype yet resulted from a cross between homozygous dominant (purple) and homozygous recessive (white) parents.
The white-flowered plants in the P generation must be homozygous recessive, as they breed true for the white phenotype. Similarly, the purple-flowered plants in the P generation must also be homozygous, but for the dominant trait. In the F2 generation, both homozygous and heterozygous purple-flowered plants are present due to the 3:1 phenotypic ratio.