Which element is described as a free-standing piece of scenery that can be folded in half for quick scene changes?

The correct choice is a book flat. This type of scenery is designed to be a movable, free-standing element, often used in theatrical productions to create backgrounds or partitions on stage. The unique feature of a book flat is its ability to be folded in half, which allows for efficient scene changes as it can be quickly maneuvered on and off stage. This folding capability facilitates precise positioning and storage while maintaining the visual integrity of the set.

In contrast, the other options do not possess this specific characteristic. A boss plate typically refers to a type of stage prop or accessory rather than a piece of scenery. A breakaway is usually associated with props designed to break apart during a scene, while cans would generally not pertain to scenery but rather to props or sound effects.