The correct answer is: B. A priestess who prophesied that Athens' navy would defeat Persia.
The Oracle of Delphi is a common name for Pythia, a highest priestess of the temple of Apollo in Delphi.
Her prophecy on the winning of Greeks over Persian had double meaning. The king Croesus wanted to get more territories, so he sent his question to the Oracle if he should attack Persian, the Oracle answered that he would destroy a mighty empire. Croesus thought that the Pythia refered to Greece as it was, indeed, a great empire. But Greeks lost the battle and, therefore, the mighty empire.