Which sentence exemplifies the literary device of foreshadowing?

The correct answer illustrates foreshadowing through the suggestion of an impending significant event in the characters' lives. The phrase "they never guessed that their lives were about to change forever" hints at transformative experiences or challenges that the characters will face, thus building anticipation and tension. This subtle clue serves to prepare the reader for future developments in the story, making them more engaged as they look for indications of what those changes might entail.

The other options do not effectively foreshadow future events. For instance, discovering a mysterious book can create intrigue but lacks direct implications about what will happen next. The image of a bird flying by as a storm brews may evoke an atmosphere or mood but does not suggest specific future consequences. Lastly, while an uneasy smile may create tension, it does not indicate a forthcoming event or change in the narrative trajectory. Overall, the selected sentence utilizes foreshadowing by clearly indicating that something significant is on the horizon, enhancing the story’s suspense.