Which of the Following Constitutes a Mutual Mistake of a Material

Multiple Choice

Which of the following constitutes a mutual mistake of a material fact?

A) Martha selling her house to Graham and hiding the fact that its first floor is damaged
B) Martha and Graham agreeing on the sale of a house, both referring to different houses
C) Graham buying a house from Martha without asking her details of damages
D) Martha selling Graham an old vase for $20, both of them unaware that it is a valuable antique collectible