The sentence that best adds to the central idea that a major workforce was needed is "... federal facility employing approximately five hundred employees..."
The central idea of a passage or text is the most important message the author wants readers to take away from it. Here, we know the central idea is that a major workforce was needed in Ellis Island in order to process thousands of immigrants a year. Now, we must find the sentence that best adds to that idea.
The best option seems to be "As a large federal facility employing approximately five hundred employees at a time, Ellis Island was a well-organized workforce." Here, we are told the number of employees needed in Ellis Island, which is quite a large one, similar to the workforce big corporation would have.
With the information above in mind, we can conclude that the answer provided above is correct.
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