Final answer:
In Greek mythology, Circe turned Percy into a guinea pig.
Step-by-step explanation:
The correct answer is d) Circe.
Circe is a Greek legendary divine figure with the powers of a witch or sorcerer. In Greek mythology, Circe turned Odysseus' men into various animals, including guinea pigs, when they visited her island.
Therefore, it was Circe who turned Percy into a guinea pig.