How is the last line of The Story of an Hour an example of dramatic irony?

a) "When the doctors came they said she had died of heart disease--of joy that kills." The doctors assume that Mrs. Mallard, who has a heart condition, died because she is shocked to see her husband alive and the joy of seeing him alive kills her.

b) "When the doctors came they said she had died of heart disease--of joy that kills." Mrs. Mallard believes her husband is having an affair with Josephine.

c) "When the doctors came they said she had died of heart disease--of joy that kills." Mrs. Mallard dies due to an asthma attack.

d) "When the doctors came they said she had died of heart disease--of joy that kills." In reality, Mrs. Mallard did not actually die.

The correct answer is: a) "When the doctors came they said she had died of heart disease--of joy that kills." The doctors assume that Mrs. Mallard, who has a heart condition, died because she is shocked to see her husband alive and the joy of seeing him alive kills her.

Indeed, the very definition of dramatic irony is when the audience, spectators or readers know something that then intra-diegetic (inside the fiction) characters ignore.

In this case, the author is using it to further emphasize the opposing views of Mrs. Mallard and the patriarchal society she lives in. She dies because she felt that with the death of her husband she would finally be free to be her own self and live her life in her own term however she would see fit. This was a source of very intense emotions and psychological turmoil for her. However, it turns out that the news that her husband had died were mistaken.

When she learned this, she saw her hopes destroyed and was overwhelmed by the realization. The cumulative emotional shocks were more than her heart could handle and she died of a heart attack; not because she was joyful to see her husband alive but because she realized that her life of gender normative subordination would continue until her death and such realization was just too much to bear, especially after believing that she was finally free.

The doctor's interpretation is completely erroneous since they judge her emotional response based on their patriarchal normative standards while the readers know the exact reason which happens to be in direct contradiction ton their assessment.