Final answer:
Frank's marriage to Dot, who has Alzheimer's disease, has been the most difficult challenge of his life because of the slow process of losing the woman he loves.
Step-by-step explanation:
The notable thing about Frank's marriage to Dot, who has Alzheimer's disease, is that having a wife with AD has been the most difficult challenge of his life because of the slow process of losing the woman he loves. This is option D.