What is one way in which irony surprises the reader? by emphasizing relationships by not fulfilling expectations by not creating a structure by expressing a simile

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. 

In the 1990s, people in Iron County believed that those who lived there in the 1950s were guinea pigs and victims, like the sheep. They have adopted the appellation “downwinders,” signifying they lived “downwind” of atomic tests.