Final answer:
The principle of Harmony is what governs how the elements of a composition work together to create unity and coherence in art.
Step-by-step explanation:
The principle that governs how elements of a composition work together to create a sense of unity and coherence is Harmony. Harmony in art ensures that all the various elements of a composition feel consistent and cohesive, creating a sense of completeness and pleasure when viewing the art. It considers the ways in which properties such as colors, textures, and shapes are combined to create a balanced and aesthetically pleasing whole. For instance, using similar colors can provide unity, and a careful variation can add interest without disrupting the sense of harmony.