A user-defined function and a component library solve different forms of duplication in the canvas-app estate. A user-defined function centralizes reusable Power Fx logic behind a named, parameterized function, so formulas do not need to be copied and maintained independently. A component library stores reusable canvas components that can be consumed by multiple apps and updated through a controlled component lifecycle. That directly supports the requirement for reusable UI elements across applications. A collection is an in-memory table used during an app session, not a cross-application logic package. A context variable is screen scoped and cannot standardize behavior across apps. A named formula can centralize a reusable calculation within an app, but the question’s paired requirements emphasize reusable logic and reusable UI across multiple applications; the function and component library provide the complete combination. The components should be packaged in solutions and governed through ALM so dependent apps receive tested versions. The Study Guide names “user-defined functions” and “component libraries” within its exact objective to “create reuseable components,” making B and D the aligned selections.

Study Guide reference/topic: “Create canvas apps — Create reusable components.” AB-410 Study Guide | Microsoft Learn technical reference