The correct selection is D. Because other orgs already integrate with objects and fields owned by the managed package, those data-model components should remain in a managed-package version while movable application logic and UI components are decomposed into an unlocked package. This preserves existing API contracts while improving source-driven CI/CD for the changeable application layer. From a build perspective, the implementation should be source-driven, reviewable, automatable, and modular. Engineering controls such as version control, package boundaries, static analysis, coding standards, and isolated development environments reduce defects early and make changes easier to understand, test, and promote. The rejected choices either rely on manual review alone, use the wrong development abstraction, or introduce custom complexity where Salesforce already provides a source-driven engineering mechanism. For this scenario, the decisive point is therefore the platform behavior represented by the selected option (D. Migrate all classes, Visualforce, and components from the managed package to the unlocked package. Install the new version of the managed package (objects/fields only) and the unlocked package with the other components.). Applying that rule consistently keeps the implementation aligned with Salesforce-supported lifecycle practices and makes the resulting change easier to review, validate, and operate across environments.

Study Guide reference: Building — Salesforce DX; package development; coding standards; static analysis; source control; secure development.