Answer: a. He answered the Sphinx' riddle and defeated it.
Step-by-step explanation:
When Oedipus was on his journey, he had to pass through Thebes and it was here that he encountered the Sphinx. It asked him to solve a riddle that no one else had so far been able to unless it would kill him.
Oedipus correctly answered the riddle and the Sphinx was so surprised that she threw herself into the sea and died. The Sphinx had been besieging Thebes and when she died, Oedipus was made King.