Which sentence has the correct sequence of tenses? before it became a federal prison, alcatraz island serves as the site of a military prison. before it became a federal prison, alcatraz island had served as the site of a military prison. before it becomes a federal prison, alcatraz island was serving as the site of a military prison. before it becomes a federal prison, alcatraz island served as the site of a military prison.

The correct answer is B. Before it became a federal prison, Alcatraz Island had served as the site of a military prison.

Sequence of tenses refers to the fact that appropriate tenses have to be used within a sentence. This means that if the first verb is in the past, all the following words will also have to be in the past tense. The only sentence that does that correctly is sentence B - the first verb is became (which is simple past tense) and the verb that follows it is had served (which is past perfect tense).