Final answer:
Stheno and Euryale were part of the Gorgon sisters in Greek mythology, a trio including the mortal Medusa—they were known for their petrifying gaze.
Step-by-step explanation:
Stheno and Euryale are both figures from Greek mythology known as the Gorgons. They were among the three Gorgon sisters, the most famous of whom is Medusa. In contrast to Medusa, who is often depicted as mortal, Stheno and Euryale were immortal. The Gorgons were characterized by their frightening appearance, with snakes for hair, and the ability to turn anyone who looked directly at them to stone. They were not one of The Fates, The Titans, nor The Mermaids; so the correct answer to which race they belonged to is The Gorgons.