Final answer:
The P generation would be classified as true-breeding parents that are crossed, where these original parents are true-breeders that produce offspring with the same phenotype when crossed with another true-breeder.
Step-by-step explanation:
The P generation, in Mendelian genetics, refers to the original parent organisms in an experiment. These parent organisms are typically true-breeding, meaning that when they are self-fertilized or crossed with another true-breeder of the same phenotype, they produce offspring which also exhibit the same phenotype.
Considering the options provided, the P generation would be classified as true-breeding parents that are crossed to produce offspring. This initial cross results in the F1, or the first filial generation, consisting of heterozygotes if the P generation had differing alleles (one dominant and another is recessive). It should be noted that the other options provided, such as the offspring of self-crossing the F1 generation or the offspring of true-breeding parents, represent subsequent generations (F2, F3, etc.), not the P generation.