Which Principle of Package Design best applies to the structure described: Modules are laid out in three layers with dependencies flowing in one direction?

The principle that best applies to the described structure, where modules are organized in three layers with dependencies flowing in one direction, is the Stable Dependencies Principle. This principle emphasizes that components at higher levels of abstraction should depend on more stable components at lower levels, minimizing the impact of changes over time.

In this context, having a three-layer structure with unidirectional dependencies ensures that the system remains stable; changes in lower-level modules are less likely to force changes in higher-level modules. This design approach promotes modularity and allows for easier maintenance, as the layers can evolve independently without causing a ripple effect through the entire system.

The other principles, while important in their own rights, do not apply as directly to the described scenario. The Acyclic Dependencies Principle focuses on avoiding cyclic dependencies altogether, rather than emphasizing directionality in dependency flow. The Atomic Design Principle pertains to breaking down a design system into smaller parts, which doesn't specifically address the dependency flow and layering. The Common Closure Principle is concerned with grouping related classes that typically change together, but it does not inherently enforce the unidirectional flow of dependencies that is foundational to the Stable Dependencies Principle.