Irony, by its most simple explanation, is when the opposite of intention or of what was expected happens. Irony, too, can be described as an instance where one “shoots themselves in the foot,” so to speak. For instance, it would be ironic if one installs a smoke alarm or fire sprinkler system in one’s house in order to prevent a house fire, but because of an electrical short, the smoke alarm/sprinkler system ends up burning the house down. With that in mind, one way in which irony surprises the reader is by not fulfilling expectations.
The answer is 'by not fulfilling expectations'. The other options don't really surprise the reader, and if the plot is going into one direction, and then what you'd expect to happen doesn't actually happen, you are surprised, and the element of irony is achieved.
