Which sea nymph was beloved by Polyphemus?

Galatea is the correct answer as she is the sea nymph famously associated with the Cyclops Polyphemus in Greek mythology. The most notable story involving Galatea and Polyphemus comes from Ovid's "Metamorphoses." In this tale, Polyphemus falls deeply in love with Galatea, but she does not return his affections, instead being in love with a mortal named Acis. This love triangle ultimately leads to tragic events, underscoring the themes of unrequited love and jealousy that are prevalent in many mythological narratives.

Thalassa refers to a primordial sea goddess but is not specifically known for a story involving Polyphemus. Amphitrite, who is also a sea goddess and the consort of Poseidon, has no direct connection to Polyphemus in myth. Thetis is a sea nymph and the mother of Achilles, but she, too, does not share a romantic link with Polyphemus. Thus, Galatea stands out as the key figure in the context of Polyphemus's affections, making her the correct choice.