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.

b. Before it became a federal prison, Alcatraz Island had served as the site of a military prison.
This is the only grammatically correct sentence. First of all Alcatraz Island has already become a federal prison. So the first sentence has to  be in the past tense. And when an incident happens before an incident in past tense, it has to be put in past perfect tense. So the second sentence in this option is also in the correct tense.

When you compare two things without using like or as thats what it means