The answer to your question is choice b, situational irony. Situational irony is when something unusual or unexpected happens in a story. For example, if you bring an umbrella and there is sunshine. You expected rain, and you got sun instead.
Dramatic irony, on the other hand, is when the reader knows something that the characters do not. For example, let's say that the book opens with a murder. The reader knows who the murderer is, but the detectives investigating the case do not. The detectives will continue to investigate and follow leads, but the entire time the reader already knows who the culprit is.