The opportunityService and opportunityServiceTest classes are in package A but are used only in package...

The correct selection is C. When Apex classes logically belong to package B, source ownership should be moved to package B and new package versions created. Second-generation packaging treats versions as deployable states, so component ownership changes must be reflected through properly built and tested versions rather than duplicate class names. 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 (C. Move the classes of package A to package B and create new package versions.). 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.