Who guarded the entrance to Hades?

The correct choice is Cerberus, the three-headed dog from Greek mythology that served as the guardian of the entrance to Hades. Cerberus is typically described as a fierce creature that prevented the dead from leaving the Underworld and the living from entering without permission. His presence symbolizes the boundary between the living world and the realm of the dead, illustrating the ancient Greeks' beliefs about life after death and the importance of respecting that boundary.

The other figures mentioned are notable in their own right but have different roles within mythology. The Minotaur is a creature that is part human and part bull, known for guarding the labyrinth on the island of Crete. The Sphinx is known for posing riddles and guarding the entrances to cities, rather than specifically the realm of Hades. The Hydra, a multi-headed serpent, is famous for its regenerative abilities and is associated with the labors of Hercules rather than the underworld.

Cerberus's role as the sentinel of Hades makes it the most appropriate answer to the question about who guarded the entrance to the realm of the dead in Greek mythology.