Which god was the father of Achilles?

The father of Achilles is Peleus, who is a significant figure in Greek mythology. Peleus was a mortal king of the Myrmidons and married the sea nymph Thetis, who is Achilles' mother. Their union is notable, as it was celebrated with a grand wedding attended by many gods and goddesses, which in itself became a catalyst for the events leading up to the Trojan War. Achilles, as one of the greatest Greek heroes in the Trojan War, often invokes his lineage from both mortal and divine sources, yet it is Peleus who is recognized as his earthly father.

Other potential choices like Zeus, Poseidon, and Hades represent powerful gods from the Greek pantheon but do not have a direct relationship as Achilles' father. Zeus, although a prominent figure, is not related to Achilles in a paternal sense. Poseidon, the god of the sea, and Hades, the god of the underworld, similarly do not fit into the role of Achilles' father. Thus, recognizing Peleus as the correct answer highlights the important connection between Achilles and his family background in the context of Greek mythology.