Answer:
The correct answer is option D. "The plants had two different alleles for the same trait".
Step-by-step explanation:
An organism that is heterozygous have two different alleles for the same gene, having only in its phenotype the trait that is expressed by the dominant allele. In this case, Mendel's F1 generation of pea plants were heterozygous. This means that all the plants had two different alleles for the same trait, but also, all the plants have only the dominant trait expressed.