The correct option to fill the gap is "had been." The sentence is in the past tense, and the action of the robbery was completed before the time of the statement. Therefore, the past perfect tense "had been" is the most appropriate choice to indicate that the robbery occurred at some point in the past and was already finished by the time the trader made the statement. The other options are not suitable to fill the gap. "Has been" and "is being" are both in the present tense and do not reflect the past event of the robbery. "Was being" is in the past continuous tense, which implies that the robbery was ongoing at a specific time in the past, rather than being a completed action.