In modular software design, which statement best describes the role of an API contract?

• A. API contracts enable predictable integration between components
• B. API contracts are optional in modular design
• C. API contracts only matter for public APIs
• D. API contracts define UI layout

Answer: (A)

API contracts formalize the interface between components, specifying what data is exchanged, how it’s formatted, the allowed sequences of interactions, expected behavior, error handling, and versioning. When these expectations are spelled out, each side can depend on a stable surface and evolve independently, making integration predictable and reliable in a modular system.

This contract approach isn't optional—without it, changes in one component can ripple unpredictably into others. It applies to both internal and public interfaces, since any interaction between modules benefits from clear, agreed-upon rules. And it isn’t about UI; UI layout is a presentation concern, while API contracts focus on the programmatic interface and its behavior.