Answer:
B. The inhabitants of Atlantis, the setting of an ancient legend, were half god and half human
Explanation:
A nonrestrictive clause must always be separated from the rest of the sentence by two commas, one placed at the beginning and the other placed at the end. An example of this can be seen in the option selected above, where the "setting of an ancient legend" for being a nonrestrictive clause, was separated by two commas.
A nonrestrictive clause is a part of the sentence that shows additional information, which enriches the text, but is not necessary to promote understanding and does not interfere in the construction of the sentence.
