Otus and Ephialtes were the sons of Poseidon and what woman?

Otus and Ephialtes, known as the Otusian giants in Greek mythology, were indeed the sons of Poseidon and a woman named Iphimedeia. She was described in mythology as a figure connected to the earth and the divine, which aligns with Poseidon's own power over the sea and the earth. The story of Otus and Ephialtes highlights their incredible height and strength, which they used to challenge the gods, particularly seeking to reach the heavens.

The other figures mentioned—Athena, Circe, and Calypso—are mythological characters associated with different stories and themes. Athena is known for her wisdom and warfare, Circe for her sorcery, and Calypso for her role in Odysseus's adventures. None are linked to the parentage of Otus and Ephialtes, making Iphimedeia the correct answer in this context.