we know that
Gregor Mendal used a process of cross-pollination,by using parents that had different traits in his experiment with pea plants. He was studying the difference between tall and short pea plants, and how genes were expressed differently between the two, so this method of cross-pollination was vital in order to accurately recreate how this process would happen within nature.
therefore
the answer is the option
b.cross-pollination, by using parents that had different traits