The authors of the passage are propounding the idea that scientists are human and therefore not infallible. Their ideas are the product of a combination of factors—not just of endless gathering of data. Mendel epitomizes the “humanness” of the scientist because his ideas about genetics were a result of his intellect (he was trained in mathematics and the natural sciences), intuition (he had an intuitive suspicion that there might be mathematical relationships among plants in different generations), and luck (he happened to use traits that produced patterns). This combination of human traits makes Mendel the epitome of the “human” scientists, making D the answer.