The type of irony is letter b. dramatic.
Explanation:
Dramatic irony is related to a situation in which the readers know and understand something that the characters themselves don't.
"The Story of an Hour" is a short story by author Kate Chopin. The main character is Mrs. Mallard, who has a heart condition that may kill her in case she gets shocked or surprised. Her sister Josephine and a friend are very careful when giving her the news that, according to a couple of sources, Mr. Mallard has died.
After crying for some time, Mrs. Mallard locks herself up in her room. The dramatic irony in this situation comes from her sister's fear that Mrs. Mallard will "make herself ill" by grieving alone. What the sister doesn't know but the readers do is that Mrs. Mallard is no longer grieving. After looking out the open window in her room, she realizes she is about to be free. Even though she loved her husband dearly, she will be able to decide things for herself from now on.