Final answer:
Option C correctly uses commas to set off a nonrestrictive clause: 'Plato, who was a Greek philosopher, wrote the tragic story of the ancient civilization Atlantis.'
Step-by-step explanation:
The option that correctly uses commas to set off a nonrestrictive clause is:
C. Plato, who was a Greek philosopher, wrote the tragic story about the ancient civilization of Atlantis.
- Nonrestrictive clauses are not essential to the meaning of the sentence and can be removed without changing the sentence's basic meaning.
- They should be set off by commas.
- In this sentence, the information that Plato was a Greek philosopher is additional information that does not change the overall meaning of the sentence, so it is nonrestrictive.