In Shakespeare's Macbeth, Lady Macbeth knows her husband possesses the ambition to want the Scottish throne, but she is afraid he does not possess the personality traits necessary to go through with what it takes to achieve it (kill Duncan). In a famous speech she worries that:
...thy nature;It is too full o'th'milk of human kindnessTo catch the nearest way [assassinate the king]. (I:5)Thus, she worries that he is too kind. She also worries that he doesn't possess the "illness" (evil) to do it. He would rather do it "holily" than to use deceit.
In this speech she also introduces an intricate personality trait of her husband's. She says that he won't want to do it, even though, once it's done, he won't "wishest [it] should be undone." So it's not necessarily the event that bothers Macbeth, it's the doing it himself.
Answer:
B) AMBITION
Lady Macbeth fears her husband lacks the AMBITION to become king
