Answer:
c.) her husband had been traveling by train and there was no railroad disaster
Explanation:
This type of irony is situational irony-- the outcome of the situation is totally different from what people expect. The real outcome is revealed in the ending when Mr. Mallard comes home and causes Mrs. Mallard to die of "heart disease." That itself is also irony (dramatic irony), but it isn't part of the question so I'm not gonna delve into that.