During his experiments, Mendel used the pea plant as an object of study. Some of the reasons were that these plants have genetic variability (different color, shape or size), crossing between varieties is easy, and self-pollination can be prevented by removing the anthers of the flowers.
Since Mendel's experiments did involve dominant and recessive alleles for a trait, and flowers can be easily protected from foreign or self-pollinated pollen, the answer is D (a plant that has only 2 alleles for a trait), since the traits that Mendel evaluated as color or shape had two alleles for each trait (color: yellow/green, shape: smooth/rough).
