"The contrast between what is expected or assumed, and what actually happens" is: foil, foreshadowing, exposition, irony, or none of these.
The literary element which is "the contrast between what is expected or assumed and what actually happens" is irony. In literature, irony can take three distinct forms: verbal, dramatic, and situational.
Verbal irony is often called sarcasm, which happens when what one says is not really what one means. For example, a reluctant student might say, "Oh yeah. I love school." Clearly what he says, his words, are a contrast to his actual sentiments.
Dramatic irony is the contrast between what a character knows and what the reader or viewer knows. In Oedipus, the main character calls down curses on the person who is causing their town to wither. The audience knows, but he does not, that he is actually cursing himself. Visually, this is the kind of irony we enjoy when we see a person with lots of packages about to slip on a banana peel. Though it's not really a very funny experience, the fact that we know it and he does not is dramatic irony.
Situational irony is the difference between what is expected and what actually happens. Perhaps a baseball pitcher is being extra careful of his arm before a big game and avoids even lifting a water glass to take a drink--and then slips and breaks his arm in the shower. In "The Interlopers," the two main characters are sworn enemies who finally make their peace--and then they're attacked by wolves.
Any time you see such a contrast, it's probably one of these forms of irony.
Of the choices that are provided, irony is the answer you want (third choice). Irony is the difference between what you expect to happen and what actually takes place. There are different kinds of irony (verbal, situational, dramatic).
An example of irony, in general, is that John, a criminal, tries to frame Walter for murder by planting a gun in the Walter's apartment; later the gun is used to kill John, the criminal.
The same exists in a fireman's house burning, a policeman being robbed, or a lawyer being sued. These are all examples of irony.
(As to the other literary devices mentioned: foreshadowing hints at events that will occur later in a story; exposition is another word for "introduction," and a foil is a "secondary" character who is in the story for the purpose of comparison usually with the main character, in order to provide—by comparison—a clearer understanding of the [main] character.)
Hope this helps.