Answer:
B) to foreshadow the ending and make it believable.
Step-by-step explanation:
This is most likely the reason why Kate Chopin decided to include this detail about Mrs. Mallard. In these lines, we learn that Mrs. Mallard had a heart condition. This detail becomes more important later on in the play, as Mrs. Mallard's heart condition will cause her downfall later on in the story. This is an example of foreshadowing, which is a literary device in which the author gives us a hint of what is going to happen later in the story.