Option D is correct because the failure is an authorization problem: every external customer can receive sensitive information even though only some customers are permitted to use it. Role-based access control assigns permissions to defined customer roles and requires the API to evaluate the caller's role before returning protected fields or invoking sensitive model functions. Properly implemented, RBAC supports least privilege and separates ordinary customers from approved sensitive-data users. Option A may route customers to different models, but model selection does not itself enforce who is authorized to receive data. Option B changes model behavior and training data, yet retraining cannot replace an access-control decision at the API boundary. Option C may improve network isolation, but placing the model in a VPC does not stop an authenticated external customer from receiving information that the API exposes. The organization should also apply field-level filtering, deny-by-default policies, logging, and authorization testing. NIST's RBAC model assigns users to roles and associates each role with permitted privileges, directly supporting this control.