Why is Marie surprised that Mersault's mother died the day before? He is not busy with funeral arrangements. He is going out with her and enjoying himself. His apartment has none of his mother's belongings. He seems too young for his mother to have passed away.

Answer and explanation:

Marie is surprised that Mersault's mother died the day before because he is still going out with her and enjoying himself and his life as if nothing had ever happened. Giving the unfortunate circumstances that took place, he acted with absolute indifference about it.