Answer:
Of all his friends (Jack, Mia, Louis), Jack was the only one he had known since kindergarten.
Step-by-step explanation:
The phrase "Jack, Mia, Louis" is considered nonessential. This means that it does not change the meaning of the sentence if it is removed. The phrase is correctly set apart from the sentence by a pair of parentheses. Commas and dashes are other ways to set off nonessential phrases.