The Bastille Prison, where many important people . . . . . . . . at one time imprisoned, . . . . . . . . today visited by tourists.

In the sentence, "The Bastille Prison, where many important people were at one time imprisoned, is today visited by tourists," the past tense "were" is used to indicate a specific event that occurred in the past – the imprisonment of important people. This event is not ongoing; it happened at a particular point in history.

The present tense "is" is used to describe the current state or condition of the prison – it is visited by tourists today. This tense choice emphasizes the present reality and the continuous action of tourists visiting the prison as a result of its historical significance.

Therefore, Option C: were/is effectively conveys the sequence of events from the past (imprisonment) to the present (tourist attraction) in a grammatically correct and coherent manner.

In summary, Option C: were/is is the most appropriate choice as it accurately represents the past event and its impact on the present state of the Bastille Prison.