(A) was registered
The correct option is (A) was registered — a passive voice construction in simple past tense, fitting "a case" as the subject.
The subject "a case" is singular and the action (registering) happened in the past, requiring simple past passive: was + past participle (registered). Options (B), (C), and (D) are grammatically incorrect — (B) omits "to be," (C) uses plural auxiliary with singular subject, and (D) omits "be" after the modal.