What literary device highlights a contrast between what is expected and what actually happens?

Irony is the literary device that emphasizes a contrast between expectation and reality. It occurs when what is anticipated turns out to be significantly different from the actual outcome. For instance, in a situation where someone prepares extensively for a rain-free picnic and then faces a sudden downpour, the disparity between the expectation of a sunny day and the reality of the storm exemplifies irony. This device can manifest in various forms, such as verbal irony—where someone says the opposite of what they mean—or situational irony, where actions have an unexpected effect.

The other options—metaphor, simile, and alliteration—do not serve this particular purpose. Metaphor and simile are both figures of speech used for comparison, helping to illustrate ideas by directly relating them to others, but they do not inherently create contrast between expectations and reality. Alliteration, on the other hand, is a stylistic device that involves the repetition of consonant sounds to create rhythm and enhance the flow of text, rather than playing a role in highlighting contrasts. Therefore, irony stands out clearly as the correct choice for showcasing contrasting outcomes.