Final answer:
Cadmus is the mythological figure who killed a serpent to found the city of Thebes, after being guided by the Oracle of Delphi and following Athena's advice to sow the serpent's teeth into the ground.
Step-by-step explanation:
The man who is known for killing a serpent to found the city of Thebes is Cadmus. In Greek mythology, Cadmus was instructed by the Oracle of Delphi to follow a cow and build a city wherever it lay down. The cow stopped in the region that would become Thebes, and there, Cadmus encountered the serpent, a guardian of a sacred spring and an offspring of the god Ares. Cadmus killed the serpent and, following the advice of the goddess Athena, sowed its teeth into the ground. From these teeth, armed warriors called the Spartoi emerged, who helped Cadmus establish the city of Thebes.